I've been using Samurize for little over a year now, and if I ever had any doubts that this program would evolve into the system monitoring tool #1, they are long gone by now.
The plugin (C++, PowerBasic, Delphi) and script (VBScript, Javascript, PythonScript, PerlScript) support makes this program even more useful.

With the default meters, you can display almost any information about your machine (or another machine via LAN). HD space, RAM usage, CPU usage, uptime, Upload/download speeds, processor speeds, fan speeds, temperatures, you name it.

Good tutorials can be found at www.samurize.com, and combined with the forums at said site, you'll always have somewhere to go with your questions.

Give it a try, you won't regret it.