Devcheatsheet catalogs over 1,600 cheat sheets for developers and software users
Non-capitalized website devcheatsheet is a pretty good place to rummage around if you're on the lookout for a free "cheat sheet" for a programming language, markup language, or a piece of software.
Some of the "cheat sheets" linked by the site are more "cheat booklets" than "sheets", such as this Essential Ruby RefCard, which stretches on for six pages (!).
DevCheatSheet (That's how I decided to capitalize it, anyway) is fairly usable and simple. The only problem with it is that it appears to be quite banner-heavy; you get two large (300x250) banner ads on the right, then another small one for WebFaction, and then a bunch of book recommendations (read, "affiliate links") from Amazon.
Snuggled into the space in-between all of these ads, you can find quite a few cheat sheets. The categories make sense, and the site does appear to be manually curated rather than scraped together. Each cheat sheet attributes a source, and you can also see its format (HTML or PDF) at a glance.
Bottom line: If I ever feel like using a cheat sheet, this site will be one of the first places I'd go looking for it (right after tweaking my AdBlock add-on a bit).