This probably old news for regular linux users so this does not apply to you
Gnome and KDE have system monitors and you can even monitor your linux system from the command line instead of using a gui app. But thoose applications generally do not stay on top and you have to minimize and maximize them when you want to use them.
GKrellM is a system monitor with lots of skins and other add ons. Most people that see linux screenshots see GKrellM in them and want to know what it is, so here it is