Miguel de Icaza announced a new project today called MonoMac, to develop apps for the Mac using Mono and C#: http://tirania.org/blog/archive/2010/Apr-19.html
In the post, he provides links to the subversion repositories, but are there any builds of it available for download?
There is no binary distribution yet. But you can build your own very quickly:
maccore
module svn co http://anonsvn.mono-project.com/source/trunk/maccore
monomac
module svn co http://anonsvn.mono-project.com/source/trunk/monomac
monomac
folder, and type make
The sample application (found under monomac/samples/Hello
) can be launched like a standard application.