Conky is a system monitor that can display just about anything, either on your root desktop or in its own window. conky has many built-in objects, as well as the ability to execute external programs or scripts (either external or through built-in lua support).
this is a full conky with most compile options enabled:
x11, xdamage, xdbe, xft, mpd, moc, openmp, math, hddtemp, portmon, rss, weather, wireless, ibm, nvidia, eve-online, imlib2, alsa mixer, ncurses, apcupsd, i/o stats, argb, lua and the cairo and imlib2 lua bindings.
use this if you are not sure what conky flavour you need.