Another nice one is available from: http://beam.to/mco
It's a very simple 4 panel design (cpu/mem - hdd - network - mbm) so no real configuration is needed, it can sit in your tray until requested and also supports transparency.
You may also prefer a statbar like the one from http://statbar.nl it basically sits either at the top or bottom of your desktop like your normal taskbar, but it displays lots of useful information
don't forget all of these real time monitors have a resource footprint themselfs...therefore, the actual figures are distorted, since without the monitor, the figure would be different then when you have the monitor running
in addition, obviously, since they use a bit of your resources, they also slow you down a bit when running, so therefore again, I turn these down until I actually need specific information
not much, a couple of mbs in coolmons case, but something to be sure