I just learned (via the etnolinguistica.org list) that the World Atlas of Linguistic Structures (WALS) was recently made available online (here). This useful resource was formerly only available in book and CD format, and it cost several hundred dollars. It is now available for free, and in exploring the new online version, I actually found it easier to work with than the older CD version. At least on my Mac, the user interface for the CD version was fairly small, which gave it cramped feeling and made it a little pesky to use. The online version, however, makes much better use of the screen, and the layout and navigation seem improved to me.
In case you’ve never used or seen WALS, I encourage you to take a look. Basically, it represents an effort to collate typological information on a large number of languages (2500, they say), present it in a easily searchable manner, and display the results on a map. Each major typological parameter (say, grammatical number) is also accompanied by an essay, which lays out the basic definitions and distinctions involved. But the best way to know how it works is probably just to play around with it. I must admit that I find I just enjoy poking around WALS, even when I don’t have any real work to do with it. It has even helped combat my Amazonia-centric typological provincialism ;).