If you are working on a budget or just like tinkering, these programs are great replacements for Windows software. I’ve used quite a few of them (look for a * next to the ones I’ve used) and starting late July, I’ll be doing reviews of some of them.
Anti-Virus:
* AVG Free – http://free.avg.com/ww.download
* Avast! Antivirus – http://www.avast.com/
PDF Reader:
* Foxit Reader – http://www.foxitsoftware.com/
Media Players:
* VLC – http://www.videolan.org/vlc/
Media Player Classic – http://sourceforge.net/projects/guliverkli/
Web Browsers:
* Firefox- http://www.getfirefox.com
* Opera – http://www.opera.com
Email:
* Thunderbird – http://www.getthunderbird.com
Bit-Torrent Client:
* uTorrent – http://www.utorrent.com/
Anti-Spyware:
* Spybot: Search and Destroy-http://www.safer-networking.org/index2.html
IM:
* Gaim: http://gaim.sourceforge.net
* Meebo: http://www.meebo.com
Firewall:
* Comodo Firewall Pro http://www.personalfirewall.comodo.com/
This special feature, Geek Girl, appears every Monday.
PDF 
Actually, most of these aren’t open source, but freeware. The only open source there is VLC, Firefox, Thunderbird, and Gaim. The rest are free as in $0 but proprietary. Just thought someone should make a distinction.
I stand corrected.