Lets get a thread going with all the free/open source programs and scripts you know of that could be useful in web design. I will begin.

Image Manipulation
- GIMP
- Paint.net

Forum Software
- PHPBB3
- MYBB

Content Management Systems (CMS)
- Joomla
- Drupal

Social Network Scripts
- PHPizabi
- BoonEx Dolphin 7
- Pligg

Audio Recording Software
- Audacity

FTP
- Filezilla ( I dont even know if there are others. I have never looked.)


Alright thats all I could think of right now. As I come up with more then I will update. Contribute some (doesnt have to be in those categories). If you would like to know if I know of any other programs/scripts from a different category just ask. Lets try and get a great list going.