I've mentioned previously how to, and Lexy Savvides has also recently flagged many of . There's also a great app for Mac that will help you learn almost every keyboard shortcut in existence in every piece of Mac software.
It's called CheatSheet. It's free, and once installed it acts a lot like that Gmail pop-up.
Just hold the Command key for three seconds, and you'll get a pop-up window showing all of the shortcuts associated with whatever piece of software you're currently using.
Simple, smart and, like all the best utilities, it's something you kind of wish was built in to both Mac and Windows automatically.