Version:            (using Devel)
Installed from:    Compiled sources
OS:                Linux

Mac OS X introduced support for "Sheets", which are essentially window modal dialogs that appear and behave differently from regular dialogs. If you've ever used a Mac, you'll see that file dialogs don't usually pop up like floating / context free dialogs, but they instead slide down from under the menubar. They are just as window modal as other file dialogs, but they don't "pop up", and they are more visually appealing, and more intuitive in how they present their relation to the parent window. When using a sheet file dialog, there is never any doubt which window the dialog belongs to. When you move the parent, the sheet follows, while blocking the window itself.

Qt provides support for sheets on Mac OS X, and it provides the Qt::Sheet window type so that any platform can support them. Unfortunately, we cannot reasonably expect Windows to support sheets, but KWIN can.

This is one of those distinguishing features that can make KWIN stand out from the croud (GNOME and the like). And it's supposedly easy to implement. It'll hit the screenshot list on SlashDot, for sure :-).