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.