ActivityMonitor

You can track system resources by launching the ActivityMonitor and activating different items of interest.

By right-clicking into the window, you can toggle the display of all kinds of resources:
Used/Cached Memory, Swap Space, CPU Usage, Network Receive/Send, Page faults, Semaphores, Ports, Threads, Teams, Running Applications, Raw/Text Clipboard Size, Media Nodes

Below the graphics is a legend (hideable from the context menu). You can change their colors and that of the graph's background via drag&drop from any color picker, e.g. from Icon-O-Matic.

You can add more views from the File menu if it gets too crowded.

Each view has it's own Replicator handle and can thus be arranged, for example, on the Desktop.