Windows only: Prism HUD provides a heads-up view of the memory, processor power and hard-drive activity your system is using, but doesn't do it all the time. The lightweight monitor app only pops up when your system really moves.
You don't have to do much more than decide the size and placement of the panels where Prism HUD will show you the stats, if you don't want to. Prism HUD's defaults are decent for alerting you to notable activity on your system, but if you want to dig into deeper statistical read-outs, you certainly can. If all that interests you is memory usage, that's all you have to have show up, though you can decide how opaque the readouts are, the colour of the panels and whatever else is needed to fit your perfect desktop.
Prism HUD is a free download, light on system resources (about 10MB on Windows 7, matching what FreewareGenius reports), and available for Windows systems only.