www.ClassicalGuitarNow.com has a fantastic collection of guitar sheet music links, as well as a customized Google search bar for other sheet music sources.  Offerings range from baroque to transcriptions of classical guitar works.  Here are a few of the links that have been culled, comprising the more popular works for the classical guitar genre:

Dirk's Guitar Page - offering one of the largest collections of free music for classical guitar.

Jan-Olof Eriksson, a Swedish guitarist, offers a collection of arrangements/transcriptions of classical works in PDF format.

Guitarist Stanley Yates provides a small collection of free downloadable scores.

The complete works of 18th century Barcelona Guitarist Fernando Sor can be found at http://fernandosor.free.fr/SorOpusAngl.html - Web Monkey Warning:  the site is pretty ugly and the score scan quality is a bit iffy, but if you can deal with the poor eye candy, this looks to be a terrific free resource for classical guitarists.