Files and applications meant for a Mac would usually end in .sit for a StuffIt archive, or perhaps .sea for a self extracting version. .dmg is another option, mainly for OS X, that means Disk Image. ZIP archives, on the other hand, very rarely contain Mac apps, and .exe files are strictly from the Windows(or perhaps DOS) world.

That said, I've never done an application search in LimeWire. Downloading music that no one plays on the radio doesn't bother me, and videos that aren't for sale or don't exist off the Internet don't bother me, but software piracy does, particularly Mac games, since I know the names of most of the prominent Mac game programmers and read their posts on forums like this. But that's up to you.