First of all, I don't expect that something like this already exists, but I am interested in knowing how something like this would be accomplished.
I want a program that I can have running in the background that will intercept ALL keystrokes entered (I'm assuming X is running and perhaps even that Unity is also running), process those keystrokes and then send the original or modified keystrokes on to be handled as normal.
My ultimate goal is creating a sort of keymapping that more closely mimics OS X shortcut behavior. This is very difficult with a standard Ubuntu install since things like ctrl+c seem to be set in stone whereas to mimic OS X behavior, it would need to be cmd+c. Anyway, I'm less interested in discussing the why and more interested in discussing the how. I'm not adverse to writing a daemon in Python, C or whatever else might be needed (I'm also somewhat comfortable with standard UNIX apis). I am, however, completely ignorant about where to begin with something like this.
When the keystrokes are intercepted and processed, it should be able to send them back to the OS following the same interface that the strokes were originally sent with. Maybe this would have to be a patch to X itself? Or would something be able to be in place between the keyboard and X?