Safari
Firefox
iTunes
Last.Fm
Yahoo Msg
Micro*****e Messenger (only reason i dont use Adium cause it wont work through my work proxy)
iTerm
Mail
Thunderbird
Seesmic
Lotus Notes
Photoshop
Quark Express and (through fighting cause i dont like) Indesign
Not many for me but the operative word being "open", the above are my mainstays and the ones I'm generally multitasking with. I'll open/close others throughout the day of course.
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.)
You definitely have too much time on your hands if you are doing that...now how about trying that on an octo-core Nehalem Mac Pro with 32 GB of RAM? Would be interesting to see how it would behave!