Windows only: If you're tweaking your system and adding new keyboard shortcuts, it's handy to know which global shortcuts have already been claimed and which are open for use.
ActiveHotkeys is a portable Windows application that surveys your system and returns a list of global hot keys. You search by checking which modifiers and normal keys you want to include in the scan. If you want to use an ALT+ some key combination you would select ALT and then the corresponding key set you wanted to check against such as A-Z and 0-9. ActiveHotkeys would then return a list of potential combinations and whether or not they had been claimed by some application as a global hot key.
There is one shortcoming, no fault of the application's author. Windows doesn't keep a record of which hot key is assigned to which application, merely that the assignment has been made. So if you really want to use a combination already assigned, you'll have to do some digging to figure out which application is using it and if you can disable or alter it. ActiveHotkeys is freeware, Windows only. Thanks Geakz!
Active Hotkeys [Donation Coder]