I am in love with LibraryThing. The website allows you to catalog your library online. I came across it accidentally when I was doing a book-title search. It allows you to catalog up to 200 of your books for free--but I immediately sprang for the twenty-five dollar, lifetime, as-many-books-as-you-want membership. Absurdly reasonable! (No, I don't work for them...)
I have had such fun cataloging the books! And it's so useful--I have devised a code to let me know where the books are, so that (for example) if I need to see if I have a copy of Dante's Inferno at home, I just look for the tag "D" (for domus). I can figure out that I have way too many copies of Wheelock's Latin, in various editions. And (here's the most fun) I can see who else has the books I own, how many unique books I own (i.e. no one else on LibraryThing owns a copy) and other fun statistics. Of course, you can keep your library private, but what's the fun in that?
I also bought a little CueCat to scan barcodes of ISBN numbers --this speeds up the entering process considerably, although most of my books don't have a barcode as they are too old. So far I estimate that I have about 1/3 of the books in the office on campus entered, and about 1/10 of the home library done. I can sort by tag (currently I own six books on Egyptian hieroglyphs, all at school, one of which no other LibraryThingers own).
If this sounds like obsessively good fun, check it out! If not, well, I'm sure you are geekily obsessed with something else, so don't make fun of me.
Buffy: You're beginning to scare me, Giles. You need to have some fun. You know, there's this place you can go, right, and you sit in the dark, and there are these moving pictures, right, and the pictures tell a story.
Giles: Yes, yes, ha, ha, very droll. I'll have you know that I have very many relaxing hobbies.
Buffy: Such as?
Giles: Well, um... I enjoy cross-referencing.
Buffy: Do you stuff your own shirts, or do you send them out?
"Halloween," Buffy The Vampire Slayer, season 2