Favored Freeware: iStat nano

[iStat nano is a Dashboard widget which allows you to] view detailed information on CPU usage, memory usage, hard drive usage, bandwidth usage, CPU temperature, battery usage, system uptime, and the top four active processes.

Pros: iStat nano offers almost everything that iStat pro does, but presents it in a smaller and more convenient interface.

Cons: iStat nano does not offer any fan speed statistics. For fan speed statistics, try iStat pro.