In addition to Mono, you could develop Mac-specific applications using MonobjC:
http://en.wikipedia.org/wiki/Monobjc[
^],
http://www.monobjc.net/[
^].
One hint which is not very trivial: with MonobjC, execution of application is different: it replaces mono command line: instead of "mono application.exe" you run "monobjc application.exe". I used both MonoObjC and UI based on
System.Windows.Forms
and managed to develop applications on Windows, which work on Mac OS X
without recompilation. I even developed a technique to combined two such UI in one project, but this is very advanced technique.
—SA