Cocotron is a potentially exciting open-source project that "aims to implement a cross-platform Objective-C API similar to that described by Apple Inc.'s Cocoa documentation." What this means is that, in principle, Cocotron would allow an OS X Cocoa app written in Xcode to be easily cross-compiled for other OSes, particularly Windows.
Of course that in principle still leaves open a bunch of practical difficulties. The guys over at Magnetism Studio (developer of FileMagnet for iPhone) have a great account of how they used Cocotron to port their Mac FileMagnet Uploader to Windows. Of course it wasn't as easy as pressing a button and having a Windows executable pop out of Xcode, but after suitable adjustments (particularly to get rid of Mac-specific code) it did make a Windows version possible. In any case, Cocotron seems poised to make cross-platform development a much less costly and time-consuming process for Mac developers.
Cocotron itself is a free download and released under the MIT license.
[via Daring Fireball]