Here a some of my favourite links:

Education

• LPI – Exam Emulation
• LEO – Online Translation

Games

• Sudoku x 4 (Game & a Solver in C)

Software

• Mailserver and add-on software
  • Exim MTA
    – DynaStop (checking for dynamic ip addresses)
    – greylistd (greylist-daemon)
    
  • Postfix MTA
    – Postgrey (Greylisting Daemon)
  • Kerio Groupware (not free)
  • ClamAV (OpenSource AntiVirus)
  • SpamAssassin (a powerful open-source spam filter)
  • NiX Spam Blackmatches (a list of current spam sender IPs)

• Monitoring software
  • Nagios
    – NagiosExchange (extensions, plugins, etc.)

• CMS/Blog/Wiki
  • WordPress