Library Thing
It's a nifty idea. Right now, there are still some glitches: it only"recognizes" editions in LOC or Amazon, which means that the 2000 first ed. of a book only available in a 2005 printing may be nowhere to be seen; the site currently leans towards US editions, although you can access Amazon's international catalogs (this will change once Tim, the site owner, incorporates the British Library catalog); and, as other users have noted in my own comments section, multivolume editions get inconsistent treatment. Nevertheless, the site is already well worth using.
[X-posted from The Valve.]