some useful apps imo:

Codec Pack (you pick your fav)
Media player (codec pack will give you mpc, MacOS will give you iTunes) if you need it.
PhotoShop
7-zip (or IZarc or WinRAR)
BT Client
Limewire (or equivalent)
VMWare Fusion
Antivirus and Personal Firewall (pick your fav. I recommend ESET or Zonealarm)
PeerGuardian 2
Partition Magic
PDF-Viewer (or your fave pdf client like Foxit, Adobe)
MS Office 2007/2008(I know Mac OS has it own office application suite, but can't beat the tide in this case)
Notepad++
CPU-Z
....
I'll update list as I think (and am not drowsy!! lol)