For myself, almost everything on the machine (except Windows and Office which came with the machine) is freeware/Open source. Some of the good ones have already been mentioned. Others I like to use are:
1) HTML-KIT
http://www.htmlkit.com/
Text editor/HTML/PHP/general purpose coding. It's KILLER feature is the ability to edit files on servers remotely. That way I don't have to connect to the server, download the file, make the changes, save and upload the file. Saves a tremendous amount of time. Plus it has plug-ins for almost anything you can think of
2) Exact Audio Copy (EAC)
Exact Audio Copy
CD ripper/encoder
This is the Cadillac of CD rippers. It just won't give up -- put in a scratched up CD and if this can't get out the music, nothing will. Fast and does encoding, tags and everything else
3) HydraIRC
http://www.hydrairc.com/
IRC client. If you know or use IRC then you'll appreciate Hydra IRC. Nuff said.
4) BurnAtOnce
Burn At Once
Very simple CD mastering/burning software. Does audio and data CDs with ease. If you're looking for something to burn DVDs with, try deepburn - the free version
http://www.deepburner.com/?r=download
5) PDF Creator
http://sourceforge.net/projects/pdfcreator/
Free program to create PDFs. It installs a virtual printer on your machine that when you select it, the output is the PDF. Fast, reliable, just plain awesome.
Those are all the ones I can think of for now.