This is the useful shortlist (by no means comprehensive) of all the free or open source software i have experience in either using, installing, supporting or extending; and more; i would recommend you try.
Operating Systems
Microsoft (starting with Windows 95 and NT4, all the way through)
Apple Mac OSX (a recent edition to the family)
Ubuntu (along with the various distribution flavours, both server and desktop)
 
Development Tools & Technologies
Visual Studio (right from v6 through to .NET 2008)
SharpDevelop
MonoRails (along with ActiveRecord)
Ruby on Rails
Aptana
Apple XCode
 
Web Applications
Wordpress (including MU Edition)
Joomla
Drupal
MediaWiki
Moodle
Elgg
 
Productivity Applications
Open Office (MS Office Replacement)
The Gimp
Mozilla Thunderbird (email)
Apple Mail (email and calendar and TODO)
Timesheet (home rolled timesheet web app)
Google Calendar