The Gimp is pretty good for photo editing, but the learning curve is sort of steep if you've never used Photoshop. On the other hand, if you've got photoshop, you wouldn't bother using The Gimp.
- Foxit Reader - Replaces Adobe Acrobat with something quite a bit faster and one might argue safer.
- Media Player Classic or VLC - Replaces Windows Media Player
- Password Safe - Stores all your passwords safely
- TightVNC - Allows you to remote control your machine (use carefully!)
- OpenOffice - 80% of what you need from MS Office. Works great 80% of the time, but will make you pull your hair out when you need the other 20%.
- FooBar2000 - Lightweight replacement for WinAMP
- Google Earth, SketchUP, Picassa
- 7-Zip - Better than WinZip
- Putty and GVim - If you ever talk to unix machines