This site has some great flamenco guitar resources- explanations of various palos, some interesting history and a pretty good selection of falsetas. Flamenco compás is a great way to explore unconventional rhythms and phrasings, and there are some very important techniques that every decent guitarist should know (rasgueado, picado and thumb techniques, most of all). Plus the ladies love it.
Arthur Kampela is a brilliant Brazilian composer/guitarist with a Ph.D in composition from Columbia. Check out his work, especially his percussion studies for guitar – they are a perfect blend musicality and virtuosity.
Colin McAllister is one of my favorite contemporary guitarists. He has also written a very useful book – The Vanguard Guitar – that is a good introduction to difficult contemporary guitar technique.
Matthew Hindson offers many free fonts, including figured bass symbols, microtonal accidentals and guitar strings (the little numbers with circles around them). Very useful stuff.
