When I first got my MacBook Pro at the end of last year, I thought its UI was very similar to Unity (the default desktop environment in Ubuntu). Now, Unity is not 100% like Mac, but I think it's very close in many ways, especially when compared with most other distros. The top menu bar, the dock, the Settings app, etc. Here's a screenshot:
(I took the screenshot on my Retina display MacBook Pro, and unfortunately, Unity currently doesn't support HiDPI displays yet, so things look a little out of proportion. But I'm hoping that come 14.04 LTS release time, this will have ironed itself out.)
Here's another screenshot, showing the top menu:
The top menu style is quite reminiscent of the Mac UI, but with two differences:
- There is no Apple menu.
- The Alt-Tab task switcher is still window-centric (though windows are grouped by their owning application), not application-centric. This probably won't make a huge difference to you unless you're wedded to the "every application is basically MDI" aspect of the Mac UI.
Update: My boss tells me that Ubuntu 14.04 now does support HiDPI displays! Here's a screenshot of Unity (with Firefox 29 beta, that also supports HiDPI) running on his MacBook Pro 15 with Retina:
(I'll get around to updating the other screenshots soon.)