I'm definitely a big fan of iSlayer's iStat Pro widget. Whenever my older Powerbook acts a little weird, I can just pull up the Dashboard and tell at a glance what's going wrong ("Oh my, the CPU is running at 150 degrees again. Time to quit a few apps"). Now they've sent word that iStat menus, an even bigger and better stats program available to sit on the menubar, has hit version 1.2.
New to this version is process monitoring, better temperature and fan support in G4 Powerbooks (like mine), and a disk activity graph that let's you monitor disk performance over time. For keeping an eye on what's happening under your Mac's hood, there's almost nothing better, and all the iStat stuff is free.
Of course, if you do use one of their apps, make sure to send them some cash to help them keep up the good work. Always support good software, people. Just like NPR, if you don't support it, you'll lose it.