Usually Safari and Terminal at any given time unless I need to run Photoshop CS3, Final Cut, Lightroom 2, and related apps. iTunes if doing something with my iPhone, obviously.
If really, really bored, I might -- just to entertain buddies -- go to /Applications, cmd-a to select all, then cmd-o to run every single application there. And waiiiiittttttt. 😀 The Dock looks truly impressive when you do that. It's very impressive when Terminal reports 10 MB free memory! Even more so when the system is still running, although not very speedily with about 180 apps running concurrently. The screenshot of that is something to behold because the row of tiny app icons doesn't even begin to cover all the apps actually running.
(In Leopard, I had to force a power cycle uncleanly because I lost control of the 4 GB MBP. In Snow Leopard, I didn't have to do this.)