There is a large variety of free guitar lessons on this site. Some of these lessons give suggestions on how to improve you guitar playing, whereas other lessons specifically discuss certain aspects/techniques of guitar playing.

OLGA (On-line Guitar Archive) a venerable library of guitar and bass tablature has been shut down. It has been around for years, used by a generation of learner guitar players. OLGA has been taken down by legal action twice before, in 1996 and 1998, but recovered. This time it looks serious, the site has been down for a few months already.