I wish I had a list like this ten years ago.
1. Linux. Although I'm still a nooj with linux, it is changing the world. You can do anything with it. (except get paid to write it) The inexperienced like me can get Ubuntu or LinuxMint installed pretty easily. If you can't get that to work then stick to using Geek Squad.
2. Sun VirtualBox. VirtualBox lets you run a VM on your computer similarly to Parallels or VMWare Fusion. I mostly use it to try and figure out operating systems that I have never used before but don't want to take the time to install it natively. If you have a MAC then it is a must, unless you are anti-windoze.
3. IMGBurn. Want to rip or burn a disk to ISO? Use this program to backup almost any disk. This program worked where Roxio failed.
4. Daemon Tools Lite. Mount those ISOs onto a virtual drive, now you can get a netbook!
5. DVD Shrink. Backup video DVDs with good quality and a few options.
6. CDEx. Extract your audio CDs to MP3. Yet another reason to own only one Optical drive.
7. Audacity. Edit audio, including MP3s. You can convert to different formats.
8. Gimp. If you are like me, 90% of the time spent editing photos is either cropping or messing with the histogram. This program isn't Photoshop, but It's enough for me.
9. FileZilla. FTP server or client. Great if you have a modded XBOX.
10. Cute PDF Writer. Create a virtual printer that saves files as PDF. If you only use Microsoft Office then just download the Office PDF add-in here.
11. NVU. Pretty good HTML editor. At least for the one page that I created five years ago.
12. Eclipse/Netbeans. Want to program? I haven't used either of these solutions enough to say which one is better, but either one will do Java just fine.
13. Visual Studio Express/SQL Server Express. Don't say Microsoft never gave anything away. They also give away free drinks to their employees. It kind of feels like a drug dealer.
14. OpenOffice.org. Useful if you can manage without Microsoft Office. However you can use Google docs instead and won't have to install anything. Personally I think everything should be written in notepad.