Free piano chord charts can be found at Piano-Keyboard-Guide.com and MakingMusicFun.net. Piano-Keyboard-Guide.com has several different charts for different categories of chords, and MakingMusicFun.net has one printable chart that covers the 28 most popular piano chords. The charts from both websites feature pictures of a piano keyboard to demonstrate the chords.
Piano-Keyboard-Guide.com has a section specifically labeled "Piano Chords Chart." In this section, there is a keyboard chart that shows which keys should be used to play all 12 major chords. The section also has a chart for all 12 minor piano chords. The rest of the page describes how to do diminished piano chords, augmented piano chords, sixth piano chords and seventh piano chords. The website lists these chords and includes links to image diagrams demonstrating how to do most of them.
The main "Piano Chords" section of the website also has links to several charts. That section describes how to do the more advanced ninth, eleventh and thirteenth chords. There is also a "chord progression for the piano" chart available in that section.
The printable chart at MakingMusicFun.net is available in the "Print It!" section of the website among the lesson resources. The chart is specifically designed to appeal to students and beginners. The chart can be opened in PDF format. The website recommends that users try out practicing the chords with the free printable sheet music provided on MakingMusicFun.net.