htop
It is a terminal GUI app:
sudo apt-get install htop
htop
About "real" GUI versions I would suggest for some Conky utilities,
but for new OS versions it's often related to specific tuning.
ProcMeter3
Look for this this system monitor (GUI works with GTK1,2,3):
ProcMeter3
Nice utility which works also with LCD devices :)

ProcMeter3 GUI on GTK3
Psymon
Also once I tried a Psymon. This is not extremely lightweight software and written
on Qt (not GTK, and therefore should work better with KDE) though despite to this fact it works well not only with Unity and Gnome, but also in other OS, like FreeBSD, MacOS, Windows etc., because it use great python-psutil Debian package. So look it closer:
Nice post about Psymon
Psymon project site
