jQuery Cheat Sheet (PNG, 364kb)
MooTools Cheat Sheet (PNG, 111kb)
Released by Jonathan Snook.
Prototype Cheat Sheet (PDF, 155kb)
Released by kangax.
Script.aculo.us Cheat Sheet (PDF, 80kb)
Released by Amy Hoy.
Yahoo User Interface Library (ZIP file 26 mb)
Released by… well… Yahoo.
Please let me know if you have any other good cheat sheets to add to the list!