I use temperature monitor/temperature monitor lite. TML runs in the menubar, which is handy. I have it set to login. The CPU sensor the G3 has can not be read by the app, however. Apple cut support for the CPU sensors in G3s and early G4s in X, but in 9, you can read them directly with a little old app by newertech called Gauge PRO.

What TM does, however, it does pick up the temp sensor in my hard drive, an 80gb seagate, which is S.M.A.R.T. compatible. Since it sits right next to the CPU card, it's good enough. I hope all that heat won't kill the hard drive quicker...

-digital