gMail: Google Gears offline backup, with settings that are very conservative. If anything happens to my account, I'll still have a copy of everything. There's another program called Gmail Backup, but I haven't bothered investigating much.
Other laptop data: Mostly music/files, not a ton. My old laptop got stolen, and I don't miss the data massively, but it's still unfortunate. I think I had an oldish backup to get some back. The new one backs up to an external drive, also sitting on my desk. Good protection against random hardware fail - not against theft or fire. I have a few things in Dropbox, but ran out of room fast (If you want to join, use this and we'll both get more). Should pay up somewhere or use the cross-computer option catbird recommended, especially now that I have a drive lying around. In theory, iluvsheep is building a NAS thingy to store at Jay St or Buddha Lounge, but in practice I think it's still on the floor here, and is moderately inconvenient for syncing with a Mac. Also installed Prey, which may help track it down if stolen and have a TigerTag on it, in case someone finds and wants to return it.
Wallet: I emailed myself a list of card names/types, plus "report if lost" numbers, plus the names of all the other cards. For security, this doesn't include my name or the card numbers at all. Need to do: add a contact info card for emergencies. Should add a TigerTag for good measure.
Phone: Easy - it's Android, so mostly backed up. Latitude should be on, meaning I might even be able to track it down. When I dropped it before, texting my office number to it let the guy who found it get it back to me, letting me save the SIM card. I also have a TigerTag on it.
Stuff: I have renter's insurance. It's under $15 a month and I strongly, strongly recommend it. If you own a car, you may get good combo rates on car and renters insurance. Very easy to do, very cheap, and it's definitely paid off for me. I pay a tiny bit more to cover replacement cost rather than current depreciated value. This didn't help a ton with my laptop - only got the price for the older version I had, not the shiny new equivalent, but if anything happens to my books, it will help a lot. Things to do: Email myself pictures of room/valuables. Take close-ups of each shelf, so I can rebuild my book collection. Leave myself a note to photo books a couple times a year.