I'm curious if there are any tools available* that will help me map out keyboard shortcuts for the application I'm working on?

Being able to see what combinations are in use, the relationships between normal, ctrl, shift, and alt combinations, and especially what combinations are NOT in use, would be wonderfully helpful.

Please forgive me if I am missing an obvious solution, but I've had no luck searching for such a tool myself, due to every application in existence having its own keyboard shortcuts, and all of those being listed on the web.

Thanks!

*Naturally, I'd prefer free/cheap, but it can't hurt to hear about any expensive options.