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?
There appears to be some scripts that have been written to help get you set up cross compiling for the Mac; I can't say how good they are, or how applicable to your project. In the documentation, they refer to these instructions for cross-compiling for 10.4, and these ones for cross compiling for 10.5; those instructions may be more helpful than the script, depending on how well the script fits your needs.
If your program is free or open source software, then you may wish instead to create a MacPorts portfile (documentation here), and allow your users to build your program using MacPorts; that is generally the preferred way to install portable free or open source software on Mac OS X. MacPorts has been known to run on Linux in the past, so it may be possible to develop and test your Portfile on Linux (though it will obviously need to be tested on a Mac).
For Linux, there is a prebuilt GCC cross-compiler (from publicly available Apple's modified GCC sources).
Update for 2015
Embarcadero's RadStudio also supports building OSX/iOS/Android apps on Windows.
For all these options you still need a real mac/i-device to test the application.