Depends on what I'm doing. Programming it's pretty much a session of XCode, IB, and SimpleText; relaxing, it's a session of CoverFlow, full screened on one display, and iTunes full screened on the other; writing, it's WriteRoom on one screen and Ulysses on the other. Usually it's just an amalgamation of a bunch of apps.
I've attached a really messy frantic work example (my desktop is usually cluttered with apps until I get fed up and hide them) and then an example of CoverFlow + iTunes, because it's my favorite way of relaxing/studying with my MacBook.
I love this idea for a thread.