MemoryWatch

MemoryWatch is a simple application for monitoring your computer's physical and virtual memory space. It's so lightweight that you can keep it open at all times since it barely uses any CPU.

Once you run it, the application stays minimized in the system tray until you click on it. When it's opened, you see a small window showing relevant information, including the amount of free space left on your hard drive.

Its few options let you adjust the refresh time interval, have it launch when you start Windows, or keep it on top of all open windows.

MemoryWatch shows all the data both numerically and visually, displaying exactly what you need to know regardless of your level of expertise.