awesome-status populates the awesome window manager's widgets. It features CPU usage, date/time, lm_sensors (CPU temperature, fan RPMs, etc.), memory/swap information, mpd information, network download/upload, sound card information (volume and outgoing level), and uptime information. An example commented configuration file is included.
iloog is a Gentoo-based live CD targeted mainly at students and scientists. It features UnionFS filesystem support that enables you to write everywhere on the LiveCD and install extra packages, a graphical X environment through the Fluxbox window manager, many programming editors and IDEs, and a plethora of scientific applications, including Octave, Namd, Merlin, and full (La)TeX support.
wmsupermon is a universal monitoring dockapp. It can be used to monitor CPU usage, disk I/O, memory usage, swap, filesystems, space utilization, network traffic, wireless link quality, CPU frequency, CPU temperature, fan speed, voltages (without requiring lm_sensors), battery status (with actual, not "reported", discharge rate), traffic from your router, or anything else. It can open and maintain many dockapp windows with one dockapp process.