Chordbook's Guitar Chord Finder is the best guitar chord finder I've ever seen. Super-slick.

Features include capo, alternate tunings, hear it by strumming or plucking the strings, acoustic and electric sound, pick chords from a list or drag finger marker dots to create your own, right and left handed views, and chord inversions. Really awesome work.

Also check out the Guitar Scales and Guitar Tuner on the same site.