GKrellM (now GKrellM2) is a popular, themable and configurable general purpose system monitor for the X window system.
As is, it allows monitoring of the clock, processor load, active processes, disk usage, network throughput and memory usage. It also allows monitoring of email accounts, system battery for laptops, and uptime.
This feature set can be extended with a variety of plugins. The look can be customized with a large number of themes. Almost all builtins and plugins allow for defining custom commands to be executed.
The name stands for "GNU (or Gtk) Krell Monitors (or Meters). For the full story, see the homepage's FAQ.