Favored Freeware: iStat pro

iStat pro is the ultimate System monitoring Dashboard widget, consisting of 9 sections which can be minimized, expanded or closed. It features detailed information on CPU usage, memory usage, hard drive usage, bandwidth usage, CPU temperature, battery usage, system uptime, and fan speed. You’ll know exactly what’s going on inside your Mac!

Pros: iStat pro allows you to easily monitor all of your Mac’s vital statistics with a single widget.

Cons: iStat pro’s interface is slightly larger than most Dashboard widgets. For a smaller version of iStat pro, try iStat nano.