Windows only: TinyResMeter is an ultra-lightweight and spartan resource metre.
While nowhere near as flashy or configurable as Rainmeter, it does run with a much smaller footprint and as a portable application. You can display dozens of variables ranging from system up time to hard disk capacity to the number of processes and threads you have. You can change the colour of the interface and have it displayed in a vertical block or a horizontal band. For another alternative, check out Moo0 SystemMonitor. TinyResMeter is a free portable application, Windows only. Thanks Beldar!