I love the GUI of Haiku, for example it has a couple of cool features
Tabs:
Right click menus everywhere:
E-Mails, contacts, documents or anything you want in your filesystem (and not just in any silly extra program):
Dragging and dropping elements (like system monitors, calculator, volume control etc.) from other programs to the desktop and pinning them there:
And more tools and themes to rice your desktop to your likings:
It reached Beta 2 two days ago and it's definitely worth trying out (even if it needs some polish). But I think this one beats any operating system GUI-wise so far (and it isn't heavily terminal-dependant like Linux). And yes, it's its own little thing, no Linux, Windows or Mac thing (hence driver support is a little bit spotty right now).