HEADPHONES: highly important piece of your kit, but often the one that is allocated a fraction of the budget. The thing is, if you're spending hundreds or thousands of pounds on a recorder & microphones but then listening via a pair of cheap headphones that is rather wasting the benefits of all the other equipment - not to mention the fact that it will, often radically, affect the 'act of listening', the amount of time before ones ears get tired and various other subtle and not so subtle aspects.
There are, of course, a vast array of headphones on the market - 99.9% of which are designed for and tested on compressed, conventional music / audio. Only a handful (in my opinion and also based on the views of recordists who've been using them for years are are know for the quality of their listening, both critical and creative) are transparent (meaning they don't colour the sound) in a way that benefits field work and have been designed using location / environmental sound. Additionally, for those interested in extended recording / listening techniques, these few allow for ones ears to adapt to the different listening focuses involved.
Headphones to consider include:
Sennheiser HD25 (have been the 'standard' h/phone of choice for location recording for years now)
Sennheiser HD380 (my h/phone of choice for extended technique work and conventional mic-ing also)
Beyer Dynamic DT1350 (new - seem ok, but i'd still go for the sennheiser's)

If you are on a limited budget & only have £30-£50 to spend then the Sennheiser HD202 is as good as anything of that price range.

'if you walk into a shop to buy headphones & it has a mirror so you can try them on - walk straight out'