Open source is great, I really love that a open community of developers often can create better products than a commersial company. Here are my picks from sourceforge.net:

Notepad++
A great notepad alternative, specially if you are coding. Allows fast color highlighted viewing in almost any language.

Gimp
Easy to use photoshop alternative, should be able to do almost anything the big boys can. There is also a version that mimics the photoshop interface somewhere on sourceforge if you look for it.

MediaCoder
Ever needed to recode some audio/video to another format?

code::blocks
Throw in a visual assist thing and I would quit using visual studio. For small projects and with good typing skills this is great.

Filezilla server, Miranda and Gaim are also useful tools, check them out.

Other stuff that is free of charge and serves as better alternatives to popular software:
CCCP codec pack
realplayer alternative
quicktime alternative
xchat 2
Foxit Reader
uTorrent

I was bored ok? allright that should do it..