Find guitar chord charts at sites such as 8notes.com, For Dummies and Chordie.com. These sites all offer selections of chord charts demonstrating common chords and their different variations.
8notes.com offers a chord chart users can browse to see different voicings. Each chord diagram also features an accompanying sound file so users can make sure they are playing the chord correctly.
For Dummies hosts a guitar cheat sheet, indicating 24 common open-position guitar chords. The site also includes brief instructions on how to read chord charts.
Chordie.com offers a large chord chart demonstrating different formations of guitar chords. In addition to a large number of variations on common chords, the site allows users to click on individual chord charts in order to view all the different possible voicings for a chord.