People complain, often with cause, that most free software applications have ugly user interfaces, bad usability or are downright of poor quality (buggy et al.).
But we (I and some friends that use Mac OS X in a daily basis) have observed that free applications that happen to be ported to Mac, begin to increase quality in all fronts, sometimes to the point hat non-Mac versions end up improving too.
The canonical example is Adium, which is based on Gaim. It is one the best applications available for Mac these days; It is even better than native iChat; and yet it is free software.
In some cases, the overall improvement in relation to Mac support seems to be be just a coincidence (but may be not a coincidence, after all). Example: Wine/Crossover is truly usable nowadays, and they have a Mac version since around 2006. OpenOffice 3 got a native Aqua version.
Another flagship example is the WebKit. This case is quite different because Apple actually invested a lot on original KHTML. But the result is still a free software layout engine that went from almost complete obscurity to a strong position, maybe even surpassing IE and Firefox someday (considering all Webkit-based browsers together). Same process is currently happening with LLVM.
There are exceptions that confirm the rule. Some people would mention XChat as an example of application that brought a bad UI into Mac version -- while I personally like the Mac version. I cannot settle the case because I am biased here; I like XChat, be it for Windows, Linux or Mac, and I like it with a dark background!
I must agree that Firefox does not feel like a true Mac application, it feels completely off compared to Safari, while Firefox in Windows seems to feel even better than IE.
But, in my opinion, GIMP is the canonical example of a bad port to Mac. It has a bad UI in Linux already, but manages to make it worse in Mac. (I know, I know, I should be using Photoshop, but I don't want to pay top dollar just to change size of a couple images per month. Thank God that Mac has iPhoto that can do basic photo retouching.)
