I just found this and think it is awsome - infact I didn't even know this feature existed! Feel free to paste your lesser-known keyboard shortcuts for everyone to enjoy - be sure to include the application!
Adobe Bridge CS4 (Windows): Alt+Delete = Add a red 'Reject' label to the image. You can then click View->Show Reject Files (toggles on and off) to hide the rejects. If you're like me and hate deleting files - this is a great way to hide them without resorting to other metadata and filtering tricks! It also shows up as a 'Reject' star under the Ratings filter
Looking forward to learning new keyboard shortcuts - whatever makes life easier makes me happy