Flexbox Cheat Sheet
by Chris Coyier
Sean Fioritto has some great flexbox content lately with this cheat sheet and this fun interactive video thingy.
If you can't get enough flexbox, there is a new video by Ryan Seddon on it and Solved by Flexbox by Philip Walton is loaded with real-world-y examples.
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*May or may not contain any actual "CSS" or "Tricks".