I'm not a musician so I don't know if these are any good but were listed in the London Guardian as "six of the best":
Rock: http://www.harmonycentral.com/Guitar
Jazz: http://www.musicweb-uk.com/
Classical:http://www.guitarist.com/cg/cg.htm
Folk:http://www.celticguitarmusic.com/celticmain.htm
Blues: http://spot.fho-emden.de/guitar/intro.htm
Martin Taylor-the boss: http://www.sonymusic.co.uk/martintaylor/
RtS