OMNI-DIRECTIONAL MICROPHONES: the pick up pattern of omni's is spherical, so the microphone gathers sound from all around the capsule. You can get 2 types of omni microphone - small omni's and conventional size 'barrel' style omni's. Whether you have an entry level hand-held recorder or a professional one, getting a pair of small omni's is the quickest way to extend the range of your set-up. 

If you're recorder is only equipped with a mini-jack input socket that has 'plug-in power' you're restricted to omni's that have a mini-jack and don't need phantom power (P48). If your recorder has XLR inputs then go for omni's that have XLR's and require P48.

small omni's are particularly good for field recording as they have a warm, decent low frequency response sound and reject handling noise. With the well put together ones you can, more or less, shake them around whilst recording and not hear the handling noise at all (though the effect of the sound might make one wobble !). They also let you get your mics in some interesting places that a barrel-design mic wouldn't fit into.

There are decent quality lower cost pairs of small omni's on the market and also mid-price range and very, very good ones at a higher cost. Amongst the better ones are those produced by:

budget:

Micbooster / Fel - clippy mics with 3.5mm jack (currently £68 a pair)
Micbooster / Fel - xlr em172 with xlr jacks (currently £115 a pair)

* these are the ones I recommend of the lower cost pairs on the market

MM Audio - BSM9's mini-jack/plug in power ($115 a pair)

Naiant - X-X xlr / p48  (nb. I no longer recommend these as Naiant have taken the decision not to supply customers outside if the USA / Canada)
Lom also have 2 designs which are ok, though I find them rather thin sounding compared to the Fel's

in-ear design:
Soundman OKMII (depends on currency fluctuations and which country one is in - around £200 a pair most places)
Sound Professional binaural pair - with 3.5mm jack (currently around £60-£80 a pair)

nb. there are an increasing number of other folks building and selling small omni's mics, most using the same capsules. The ones above are suggested as they offer good quality, professionally built products, good customer service and have a good reputation. Some of the other brands have issues with their products or attitudes. 

mid-range:

Rode Lavalier - can be used with mini-jack or xlr (£150 each / £300 a pair)

nb. there are also other makes at this kind of price level - the Rode are suggested as they come with its own, clip-connect windjammers with a built in 'space' and again most of the others in this price range use very similar capsules but can cost more. However personally I think most users will find the Fel's stand up to the Rode's well.

the gold standard:

Sanken cos-11 - p48 required (approx £300 each)

DPA4060 - p48 required / xlr and various other connections for pro-recorders / mixers (price fluctuates due to exchange rates, but usually is around £6-800 for the stereo kit)

DPA SMK4060 stereo kit

Sanken COS-11

Rode Lavalier

MM Audio BSM9's

Naiant x-x omni

having tested and used just about every small omni on the market the DPA's are, quite simply, some of the best sounding mics you'll hear and whilst they might appear expensive to some, when you consider that they sound as good and indeed better than mics costing several thousand £'s they're actually very reasonably priced. 

nb. all prices quoted are UK prices and include possible import charges for the ones that can only be purchased from overseas.

You'll need some form of wind protection for your omni's: you can mount them inside a standard blimp (wind protection cage for microphones) or you can buy small fluffy windjammers. As with all windjammers try to avoid the really cheap ones on ebay and other sites as they're usually just fake-fur rather than proper acoustically transparent material and they'll muffle your sound to various degrees. Of the 'best' ones there are Rycote (approx. £30-£35 a pair) and Bubblebee Industries (approx. £28 each). Personally I use the BBi's as a) they stay on ! (the Rycote's tend to fall off quite easily) and b) they have small bubble inside providing space around the capsule, which also helps with protecting them from wind-impact noise. 

of all the 'barrel' design larger omni's there are, as usual, lots to choose from. If you're on a budget or have a medium amount to spend Rode tend to offer decent mics for the price, as do Pearl, Oktava & some folks also like sE and other lower cost brands. Personally I find Rode and Pearl offer the best value for money at these price ranges. Moving up in terms of cost it is true that there are certain brands that are 'safe buys' - such as DPA, Sennheiser, Neumann, Sanken & Schoeps - these can range from £1000 up to £4000 per mic. 

Omni capsules are often used in Surround / Ambisonic mics - more about this in a future post.

TECHNIQUE: as with all microphones, the craft of using them is (and should be imo) personal. One can read countless books and web pages on the placement of any microphone but getting out there & experimenting is really the only way to learn what works for you and what doesn't. Two particularly good placement techniques for small omni's are binaural and 'spaced omni's'. 

SPACED OMNIS: here the two omni mics are clipped to either the mounting rings of a blimp or some other bar / mount (including oneself, a bag / rucksack or indeed the classic Watson-style coat hanger - particularly useful as it comes with its own handy handle / ground spike). The sound you'll get using this technique has a great sense of space and sound localisation - very 'open' and immersive for the listener.

Omni-directional microphones have a good low end response and therefore can have (depending on which ones are used) a general sound profile that is nearer to the sound we hear with our own ears - hence it has the ability to immediately tap in to your audiences subconscious listening process.

Unlike ‘binaural’ recordings, spaced omni recordings can be played back through loudspeakers, from stereo to ambisonic and multi channel.

You can place a baffle between the two mic capsules and there are various techniques for doing this, for example the classic ‘Jeckin Disk’. Some of these techniques aim at an accurate ‘binaural’ recording and others play with the effects of sound on locations.

BINAURAL: for more information on this technique see: 

http://jezrileyfrench-aquietposition.blogspot.co.uk/2014/09/binaural-this-is-recording-technique.html

Put simply, a pair of decent omni microphones is a very good addition to any recording kit and is rather useful for those starting out who want to extend the reach of their hand held recorder.

in addition:
just to clarify a few things: DPA's (and indeed most decent omni's) don't need to be in a suspension really - they're omni's so they reject handling noise (another good reason to get some !).

Mounting them next to ones ears is one technique yes but the most common use for them across all sectors of field recording / location recording is as personal mics in sound crew work & next to that its as spaced omni's for wider field recording applications.

I’ve heard a couple of people mentioning that using them next to your ears will give you 'sort of' binaural but actually it is full, proper binaural- there's a slight difference of opinion amongst recordists as to whether in ear or out of ear works best for binaural but both are fully binaural techniques. As I said there is a difference of opinion but personally (& i'm not alone in thinking this) out of the ear always sounds more natural - with the in-ear design you are blocking some angles of pick up for the mic and with our ears sound from every angle enters our pinar & psychologically we 'fill in' the missing parts between our ears - therefore with in-ear binaural recordings we tend to hear it as hyper-reality & we perceive it as 'effect', which means we don't connect to it in the same way.

If you want to mount them as a spaced pair with a jecklin disc between, the SMK4060 kit comes with 2 pressure zone rubber circles & one of those works quite well as a light, practical and free jecklin disc (free because the cost of the stereo kit is cheaper than buying two separate 4060's). As mentioned in the article at the above link, a very good technique for mounting them is by clipping or taping them to either end of a coathanger - this allows you to then bend the 'arms' of the coathanger in if you want to alter the spacing, use the hook to hang them from a tree or other point, straighten the hook out & use it as a ground spike etc etc. Its a method lots of recordists use inc. Chris Watson, who started using this technique some years back. 

There’s a myth about the 4060’s that is often stated by some nature recordists, who claim that the self noise rating of them means they are unsuitable for nature recording - this is incorrect. Yes, they have a higher self noise rating than some other mics but they’re also more sensitive, meaning the gain doesn’t need to be turned up on ones recorder. Also, the simple fact is that the 4060’s sound more transparent than lots of other mics & so they sound much better. As i’ve often said one should never choose any bit of equipment on the spec sheet - its how it sounds that really matters. Anyway, when one considers that, for example, Chris uses 4060’s (along with other mic arrays depending on the situation) on quite a lot of his film / tv work inc. the Attenborough programmes it does indicate that they are rather good for recording nature !

The cost of the stereo 4060 kit is currently around £610 (dec.2014) which might sound a lot to some folks - but when one considers that they compete (& win) with mics costing several £1000 each they’re actually very good value indeed. However there are alternatives at different budget levels - they might not be as good in terms of how they sound & self noise, but they're still an excellent way to extend your recording kit.