Shortcut keys mapper viewer/checker available? Is there a tool or method to check what shortcut key is mapped to what?

I used Command-Space for special purposes, after installing this and that, and changing this and that, when I click Command-Space, I see some black screen showing 'No items selected'. It seems that the Finder or something hooks the key, but I'm not sure.

Any idea?

I find that Key Cue shows the assigned shortcuts for everything. I wish, I can get the same information.
Try System Preferences->Keyboard and look at the Keyboard shortcuts tab