Mac: Learning keyboard shortcuts is one of the best ways to enhance your productivity, but there's so much to learn, especially when you count individual applications' own special shortcuts. CheatSheet is your Mac shortcuts friend: the free utility quickly summons a list of all keyboard shortcuts for the app you're using.
Once you launch CheatSheet (copy the unzipped file to your Applications folder), the tiny app will wait in the background. Within any application, hold down the Command key (⌘) for about three seconds to bring up the list of all the keyboard shortcuts.
It's like holding down Shift + / (or ?) in Gmail to see every Gmail shortcut, except this is for just about any application on the Mac (I've tested it with Finder, Photoshop, Google Chrome and Thunderbird.)
Sometimes there's more of a delay to bring up the shortcut list or the app doesn't respond to the hotkey (I think this depends on how busy your system is), but you can relaunch that program, hold down the Command key again or relaunch CheatSheet.
That very occasional issue aside, CheatSheet is handy app to get to know all the shortcuts at your fingertips.