You can run .NET application without recompilation on Mac OS X under Mono:
However, those application have to be compatible. Mono supports all of the BCL and some of other non-standard FCL, notably
, ADO.NET, and more.
I must also note that, in contrast to Linux, Mac OS X is extremely hostile to the "foreign" GUI applications. They work, but look ugly. If you confine yourself to console applications, it's not a problem, but Forms look like something outlaw, even though they work. One major reason for that is the totally different concept of the main menu. You can also develop "native" Mac OS X UI using Mono and the library MonobjC which wrapes, Objective-C, Cocoa, etc, but I don't think you are interested in that. Anyway, this is a possibility.