Let's see, at work it's the basic office-y type programs (Firefox, Eudora, iChat, Word, Excel, FileMaker Pro, iCal and occasionally Toast and Apple Remote Desktop).
At home, I tend to keep a lot of things running simultaneously:
Firefox, Thunderbird, iCal, Google Earth, Photoshop, Illustrator, Roxio Jam, Sound Studio Pro 3, iTunes and Acrobat Reader/Pro.
I take the 43folders approach. Everything is closed and hooked onto a quicksilver-trigger *until* I open it. Makes me feel like I'm in a garden of serenity, or something. So, I guess the only thing I run all the time is quicksilver.