Now that printing's working and Renoise has been ported to Linux (it works amazingly well, though I still need to figure out the best way to enable realtime audio and add Windows VST plugins), I might never have to use Windows again :-)
There are still a couple of things: Skype video webcam compatibility and formatting an iPod as FAT32 in iTunes, for example, but those don't happen often. Hopefully once Jingle gets standardised we'll have audio and video in Pidgin, Adium and Google Talk, so won't need Skype. The libjingle source hasn't been updated for a year though.
There's still games, too, of course...