If you're a total geek, the iStat Pro widget just might be the ideal computer pastime. It's a system monitoring widget, that features detailed information on your Mac's CPU, memory, hard drives, IP and external IP, bandwidth, CPU temperature, battery, uptime, temperatures, and fans. In essence, you can see what's really going on inside your Mac all day long.
Download the iStat Pro widget at islayer.com for free. Minimum Requirements: OS X 10.4.