Spotted this great list of open source tools for system administrators: 24 Great Open Source Apps for Admins & Technicians.

I can personally vouch for a number of these:*Angry IP Scanner

*PuTTY

*DBAN

*DeltaCopy

For some tools that are not open source but free, you can’t go past live.sysinternals.com ( details here) - now owned by Microsoft themselves these tools make a Windows Sysadmin job much easier.