LunaMenu is a simple menu bar item that displays an icon of the current phase of the moon. You can also opt to have a floating moon on your desktop, and there's a window with a detailed summary of the current moon phase.
DRDUL Excellent and simple app. Even though this version pre-dates Leopard, it runs fine on my Mac Pro and a unibody MacBook. The only feature it's missing is a visual display of moon phases in the calendar in the Info window. (Version 1.2)
EAN Brilliant, as usual. Download it, try it, love it, donate a few bucks.
The are only a few possible features left to implement. I'd like more control over the appearance of the Moon Window. By appearance I mean both how it looks and how it gets there. (Version 1.0)