A look at GNOME Shell

Speaking as someone who has used OS X quite heavily in the past, I *love* the new Alt-Tab cycling behavior. You can still get to the window you want with one hand -- Alt-Tab to the right app, and then cycle between windows of the same app (I rebound mine to Alt-`, but on my netbook which does not have a separate ` key it's Sys-Tab, like in OS X)

You get most of the benefits of a tabbed applications, even in apps that do not support tabs. And it makes Alt-Tab usable when having lots of GIMP windows open (before that I used to curse MDI apps incessantly)