Rosetta Stone for Unix is a very handy cross-index of various commands and tasks across various Unix variants. The index of tasks is especially handy as a quick 'how do I do this on X' pointer. It's available in several formats, and as a bonus you get some helpful links as well.

(Since I was just using this today to figure out how to do various things on Solaris, I figured I should finally get around to mentioning it.)

(From a Slashdot comment.)