It seems to be a small CPU monitor with a Classact/Reaction GUI. Very simple, there's just a single "fuel gauge" showing CPU usage. The author claims in the guide that it's very accurate but it always seems to show at least 60% on my system even when everything is idle. However as I'm running on Amithlon with Executive it may not be a fair test.
Edit: wow, you two beat me to it while I was writing my reply!