Q:

"Mac-like" Linux distribution

I have an ancient Mac that I'm looking at wiping when Lion support goes the way of the dinosaur. I still like the idea of keeping it as Mac-like as possible. 
Are there any Linux distros that mimic to a very large degree OS X functionality? Interface bears more importance than under the hood development.

A:

Elementary Luna OS aims to be as "Mac-like" as possible. It's built for usability and to visually look like Mac OS X.

Click to view in higher resolution
Etoile is a Linux distro aimed at using the GNUStep project to do system tasks, thus allowing a vaguely similar programming environment to Cocoa. While more similar to NextStep in terms of visual style, it will have some familiarity from a programming perspective.

A:

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.)

A:

EDIT: Clementine OS is now long dead. Please disregard this answer, and use eOS or something instead.

Clementine OS aims to be as similar as possible to Mac OSX --  it boasts many of the same features, such as expose, and is visually almost the same. It comes from a now discontinued (sold) project called Pear Linux and has an interface virtually identical to that of Apple. Here is a screenshot:

If you just want an Apple-like interface, I think I will go so far as to say that Clementine OS is perfect -- Oops! I just did --, although you may not like the Mavericks-style.
Under the hood, it is based on Ubuntu, so it is nice and stable and has a lot of available packages.

