Conky is a free, light-weight system monitor for X, that displays any information on your desktop.
Conky has more than 300 built in objects, including support for:
* a plethora of OS stats (uname, uptime, CPU usage, mem usage, disk usage, "top" like process stats, and network monitoring, just to name a few)
* built in IMAP and POP3 support
* built in support for many popular music players (MPD, XMMS2, BMPx, Audacious)
* can be extended using built in Lua support, or any of your own scripts and programs (tell me more!)
* built in Imlib2 and Cairo bindings for arbitrary drawing with Lua (tell me more!)
... and much much more.