I used to use one that was super tiny (the cpu and ram bars were probably 1 or 2 pixels thick), I also used it with windows 98 (I think) so it must be pretty lightweight. I'll look through my download backups later tonight to find the name for you.
What I got now may intrest some people on this forum. It's the
Logitech G15 keyboard. It has an LCD screen that you can write applets for. One of the few applets it comes with is a CPU/RAM usage meter.
If you'd like to see othe apps written for the keyboard, or if you just want some more information; check you
www.g15forums.com.