YTClock is a very light-weight, beautiful transparent clock.
Mac version is still in early development phase and only supports basic capabilities. Use ⌃⌥⌘C to make it clickable or unclickable.
This version of YTClock is written in Objective-C/Cocoa from scratch. It's not just a port of the Windows version :)
Current version runs only on 10.5 (Leopard) or later.

There are two separate programs in the package; one is a very small core program, and another is a customization program. Both are written in C++, and the core program directly calls the Windows API to make it very lightweight and to use the latest Windows capability.
The clock is transparent both in terms of its inputs and outputs. To click the clock itself, you need to press the Ctrl key, or the mouse event will be delivered to the window underneath the clock.
Just extract the ZIP file to use the software. It won't change any registry settings, so you can uninstall the program just by deleting the files.