How often do you her people saying that they still use windows because software xyz sure doesn’t exist on linux. Now you can beat them: osalt.com lists open source alternatives to common commercial software.
I just did a quick search for the software that is responsible for my windows partition:

Instead of itunes I should use Amarok or Banshee - well, that is probably a special case because both problems won’t allow me to buy software in the iTunes store.
The other software is the “WISO Sparbuch”, a German software for my tax declaration, and unfortunately nobody has yet built a good tax software for linux.