How do I "unhide" or "show" a hidden login item on Mountain Lion? I accidentally hid a bunch of login items, some of which I meant to delete. How do I unhide these items once they're hidden?
Press command and tab to see the apps that are "running" and select the apps one by one to reveal the windows that the system hid for you. 

Alternately, you could deselect the hidden check in the log in items preference and restart (or log out and in again).