Google Docs and Spreadsheets
You can checkity check it here.
While it is by no means an "Office-killer," it does make some steps in the right direction. It adds to the whole "Google acting as my virtual PC" thing. I can't wait until you can start running applications on your virtual Google PC, or when they cut the crap and officially make your 2.7 gigs of mail space a virtual hard drive. That'll be the day.
Then maybe I can finally play some online Magic, because there's no programs for doing that on Mac. (And I refuse to emulate Windows or dual boot it...I know, I'm whiny today)