I would like to port my C/C++ apps to OS X.
I don't have a Mac, but I have Linux and Windows. Is there any tool for this?
For Linux, there is a prebuilt GCC cross-compiler (from publicly available Apple's modified GCC sources).
https://launchpad.net/~flosoft/+archive/cross-apple
Update for 2015
http://channel9.msdn.com/Events/Visual-Studio/Connect-event-2014/311
Embarcadero's RadStudio also supports building OSX/iOS/Android apps on Windows.
This answer by Thomas also provides a cross-compilation tool.
For all these options you still need a real mac/i-device to test the application.