UPDATE 3: I found the source of the popular LaTeX cheat sheet. It includes a PS version, a PDF version, PNG views [Page 1, Page 2], and the LaTeX source. It's nice that the source is available. That makes it easier to shift from A4 to letter paper and back.
UPDATE 2: More reference cards can be found in the follow-up to this message.
UPDATE 1: Check out refcards.com and the old version old.refcards.com. There are lots of useful cards, including some of the ones I linked below. This TeX card and this Vim card look familiar. There are other EMmacs and Vim cards in the editors section. There is also a typesetting section that includes a LaTeX cheat sheet.
The Brown University Center for Fluid Mechanics, Turbulence and Computation (CFM) has a collection of computing tutorials. Buried in there somewhere is a useful Plain TeX reference card in PostScript format. I have generated a PDF version of this reference card.
I stress that this is a Plain TeX reference card, and so there are different ("better"?) ways of doing some of these things using other LaTeX macros (note TeX vs. Plain TeX vs. LaTeX differences: [1, 2, 3, 4, and more]). However, this is still a very useful reference card. It prints out as two landscape-oriented pages (weirdly sized: 11.7" x 8.3"). I plan on printing this out and taping it next to my desk at the office. I also found a useful longer Plain TeX reference sheet that may be helpful to some people. Of course, there are even longer Plain TeX reference documents also available.
Oh, I also found this AUC TeX reference card PDF with available source code. I don't use AUC TeX (I favor VIM-LaTeX), but maybe someone who does could use this. FYI, AUC TeX is a package for EMacs providing extra support for LaTeX (and others).
Speaking of Vim:
- Here's a cool set of multilingual Vim quick reference cards (again, PDF with 2 landscape-oriented pages) that include their sources and an HTML version.
- Here's a similar Vim quick reference card (PDF).
- Here's yet another Vim quick reference card (PDF).
- Finally, here's VIM-LaTeX's quick start introduction document.