I've got some lib files (of boost and nvidia's cg) that im using under windows. The boost libs i've built my self under windows. I'm wondering if lib files are cross platform or if i need to rebuild boost under mac and get mac lib files for cg.

Also does mac use the .lib extension like windows, or the .a extension like linux for library files?

1: .lib is not cross-platform. You'll need to rebuild them.
2: The Mac uses .a for static libs, and .dylib/.bundle/.framework for dynamic libs (the latter two of which are simply directory structures containing a dylib and some other stuff).

To be clear, just because Mac OS X and Linux both use ".a" for static libraries does not make them in any sense compatible.

A framework contains a dylib, and a .bundle can be either a package or a single file, but "bundle" and "dylib" are not semantically equivalent on Mac OS X, unlike Linux/Windows where .so/.dll covers both.