Using iPhone apps in Mac OS X ...

They don't even use the same processor or firmware, so how would it work? There might be an iPhone hardware emulator that developers use, but this isn't what you seem to be looking for.

 

The simulator compiles your code into an intel compatible binary, the binaries distributed on the app store are ARM binaries... so no, it's not happening.

 

To do this you'll need the

1. source code of the App
2. The iphone Dev Tools(free I think ) from
3. Run the iPhone emulator, and compile the Source code as a intel binary using xCode

Ps: you have to be on leopard for this to work!

 

Yes, free, but I don't think there are many open-source iPhone apps out there. I'd be interested to check them out if there are some available. I don't know the first thing about iPhone development.