From what I have seen it looks like GeekTool is a program that is, more or less, Rainmeter on Macs. I don't think it is as versatile and have not tested it since I don't have a Mac.
If you have a Mac could you test this out and tell me how it works? I can't provide a review since I have not used it.
Hope this helps you Mac users.
Link: http://www.makeuseof.com/tag/how-to-display-system-information-in-style-using-geektool-mac/
