GNOME system monitor applet it is.
Many times when I'm thinking of using a different desktop suite, I know that I'm not gon have an equal of GNOME's system monitor applet. In Debian, this applet defaults to displaying CPU usage but can be set to display more, including memory and hard disk usage. This is among the things I add to the panel on every fresh install (for I always am curious what's up with my system). Given how useful it is, I now wonder why Debian doesn't add it on the panel by default, especially that many new users won't even be aware it exists in the first place.
Clicking on that applet launches a more comprehensive system monitor which has 4 tabs (notebooks in GTK+ speak):