Drivers:

Clean System Directory - 98/etc. - Remove all those stray DLLs from your system.

Driver Sweeper - XP/Vista/etc. - Clean out old drivers to keep your system in tip-top shape.

Hardware:

Belarc Advisor - Vista/XP/etc. - Get a comprehensive report about your hardware and softare setup.

M2 Information - XP - Free tool scans your system and records hardware specs.

PC Wizard v2005.1.64.3 - XP/etc. - Detect hardware and diagnose PC problems with this powerful utility.

SIV (System Information Viewer) - XP/Vista/etc. - Get a complete rundown about your hardware.

Windows Memory Diagnostic - XP - Check the health of your system's memory.

Performance:

Advanced WindowsCare Personal - XP/Vista/etc. - quick and easy way to juice up system performance.

Everest Home Edition - XP/etc. - Diagnose your OS, and benchmark it, with this utility.
Fresh Diagnose - XP/etc. - Benchmark your PC and check whether it lives up to the manufacturer's claims.

LocalCooling - XP/Vista - Fine-turn your power settings to save electricity.

Network Bandwidth Monitor 2 Lite Edition - XP/Vista - Monitor and graph your bandwidth use.

Ultimate Boot CD - Vista/XP/etc. - Run diagnostics from a CD, not from your ailing hard drive.