LoadInDock is by far the best CPU monitor that I've ever used. I love being able to tell exactly how much processor time is being used at any given moment. I keep it in my dock right next to the Finder icon, and I have it set up to automatically launch at login via the Accounts pane in the System Preferences.

On many occasions, LoadInDock has helped me realize that an application has hung, locking up the CPU at 100% load (after which I can use Terminal to check which process has crashed using the "top" command and end it using the "kill" command). If for no other reason, I recommend LoadInDock to help advanced users keep a more watchful eye on their system.

I primarily use LoadInDock on Mac OS X v10.3. I look forward to using it in Tiger as well (version 0.8 supports it).