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.

The Settings menu opens a panel where you can set the update interval.

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