One of my favourite genres. I love the relaxing but sometimes melancholy feel of much of the genre and often emphasis on lyrics (unlike much of my metal
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So here's my short guide to folk:
Nick Drake... released 3 perfect albums, and I honestly can't think of a bad song from him.
Leonard Cohen. Genius lyrics (easily rivals Dylan in my opinion), but very depressing. His singing can be an acquired taste for some, but I quite like it myself. Plus he made some hilariously cheesy albums in the 80's 
Neil Young
I especially like the acid/psychedelic folk subgenre
Comus... this is the most bizarre music you'll ever hear, it sounds like the soundtrack to a weird cult performing a human sacrifice in a forest (listen and you'll see what I mean.)
Roger Rodier... much more sane than Comus, but it's still pretty 'out there'. The most underrated musician in this list... thanks to Noble Savage for discovering him
Neo-folk... different from most folk, and much more of a cult subgenre. Lyrics often concern nature, paganism etc and the music is dark and not that easy to get into, but often is atmospheric. In my opinion most of it is garbage but some bands are very good.
Agalloch... mainly a metal band with folk influences, their latest E.P. is completely neo-folk and has some really beautiful atmospheric songs.
Ulver also released a neo-folk album (before they played black metal and now weird experimental electronic music)