Xosview displays many system related stats such as cpu usage, memory usage, swap usage, network usage, interrupt activity, serial activity, and load average inside of an X Window. Platforms: Linux, NetBSD, FreeBSD(alpha), HPUX, SunOS, GNU HURD.
You can see a Screen View @ http://img109.imageshack.us/img109/1738/compositeb.png (it can't be displayed here due to size limitations).
Reliable and runs smoothly
This is a no frills and low overhead System Monitor that accomplishes what is was developed for.