Well besides the 3 default ones Internet Explorer, Windows Explorer and Windows Media Player (which of course you can un-pin). Out of interest what else have you got pinned to your taskbar in Windows 7?
Personally I have added:
If you want to learn a little more about how to pin new items to your task bar watch this video.
Hmmm, I've unpinned all the default ones and replaced em with Firefox, Pegasus Mail, Visual Studio, Putty, a few links to network shares and to my downloads folder.
At home I've got: Firefox, Safari, Chrome, Opera, IE, Media Player, TrueCrypt, RealVNC, Visual Studio, my investment account's desktop app, FileZilla, Windows Explorer, OpenOffice Calc, and Thunderbird.
I like the new taskbar concept, but not being able to put these in rows and columns is going to hurt me eventually - at work I have 36 items in my Quick Launch bar with XP, and I use all of them regularly. No matter how I set my screen, there's no way I'd get all 36 of them to show.