Gkrellm

Gkrellm is easy on resources, unobtrusive yet informative, and skin-able. It's the first thing I get running on a fresh Linux install.

I just love it. It's fast, smooth and lightweight. Most importantly - it's so small and powerfull that full screen of plasmoids, desklets is not enough to beat it.

In these days, plenty of plasmoids, desklets and stuff, it's a proud to look down right there (I place it on this side) and see how the little gorgeus stack (wider, narrower, as you want) keeps on monitoring with almost any cpu load ... as previously stated, there are lots of plugins for this little but powerful app, and a lot of skins to make it match your desktop look.
You can collapse it with a single middle-click, if your screen it's not very wide. I use to have it always there, and configuring it as a dock make my other windows respect its space, so it's always visible. Another trick is clicking on the krellms (the little windows that show graphs) and see the numeric data. You can reverse or split the graph in order to see in and out traffic separately, or tell Gkrellm to draw it as a line.
It works on any desktop you choose from fluxbox to KDE. It will proudly show your system's name, uptime kernel version and more. It has reminders, alerts,... try it! This is one of the first apps I point to winusers, saying "because Linux knows how to handle memory usage, I can see the inside of my system in almost real-time ... can you? "Gkrellm it's like when you look at the dashboard of a good car. Makes you feel the control. Make me feel good. I love my desktop.

One of the best system monitors for overall system performance monitoring. With an easy to use plugin interface, it is a cinch to write new plugins for new desired monitors.