CheatSheet is a nifty little application that lets you quickly review all available keyboard shortcuts for the application that is currently in the foreground. Any keyboard shortcut that is listed in the menu of an application will show up in an overlay, sorted by menu item.
I haven't been using it for long, but for these moments when you simply can't remember that one shortcut, this app might come in handy. Give it a try, it's free.
Link over at Macworld