Mac progress: System.Windows.Forms working on top of X
System.Windows.Forms (or SWF) is an API that the .NET framework providers for writing graphical applications. There are others, like Cocoa#, Qt#, and Gtk#, but on Windows, SWF is what developers use.
If your goal is to write cross-platform .NET apps (as mine is), then getting SWF working on the Mac is important.
It's pretty easy to do, too. I ran into a glitch with it giving an exception when I tried to run my test application, which turned out to be that I didn't have an X server installed. OS X comes with one, but you have to install it (it's not installed by default). Download it here.
I understand there is a "Cocoa Driver" for SWF that would let my .NET app run without X in development, but it's not ready for prime time yet. Still, this works for now.