It's a little aquarium that sits right above the system tray and displays contents representing the system state. For example, the water level indicates how much memory is being used. If the rubber ducky in the aquarium is drowning, your system has insufficient free memory. It has many other cute indicators, like bubbles for CPU and fish for network activity.
It's a very small download and yours to keep free, for as long as you like. Don't worry - it doesn't have any spyware or adware in it either. Just good old freeware.
Thanks buddhima for sending this link.
Click here to try it: