There's GNOME System Monitor, Plasma System Monitor, MATE System Monitor, Mission Center, Conky, Resources, Stacer, Htop, Glances, and more.
Also: I've tested many portable Linux distros, but PorteuX is the one I keep on my USB driveAnd now there's COSMIC System Monitor, a lightweight system monitoring tool that integrates with the COSMIC Desktop.
As the name implies, however, COSMIC System Monitor was designed specifically for the COSMIC Desktop, so if you want to use it, you'll need to use COSMIC.
What COSMIC System Monitor monitorsCOSMIC System Monitor monitors everything you'd expect it to, including applications, processes, process utilization per core, CPU usage, memory usage, disk us…