CCleaner is great!!! Originally called CrapCleaner and it does a good job at that along with registry fixes and managing startup files too. Piriform who makes CCleaner also has a free specification viewer called Speccy that is handy.
Here are other free tools I use frequently to maintain my system:
- Revo Uninstaller-to remove all remnants of programs no longer wanted
- Process Explorer from sysinternals.com - it's like an enhanced Task Manager
- CPUID's CPU-Z and HWMonitor
- Prime95 - runs stress calculations to test your RAM memory/chipsets
- Auslogics Disk Defrag-quick and easy defragger
- Glary Utilities-good collection of tools for free!
- Driver Sweeper-extra cleaning for sound and video drivers
- Spyware Blaster-protects you from malware sites
- Google Earth
- Audacity - audio file re-mixer
- MS Security Essentials-takes up very little system resources compared to other anti-virus
I purchased Casper (backup utility) to make hot-swap hard drive clones of my system in case of a drive failure.
For those who are still active-duty or in federal service, you can sign up for MS Home Use Program and get the latest 2010 Office Suite (all programs) normally $500 or more for $19.95.