[Techowl]

I've personally gotten a lot of value out of the CSS-Tricks flexbox guide [0]. It's visual, and sufficiently comprehensive.

0 - https://css-tricks.com/snippets/css/a-guide-to-flexbox/

[toxican]

I second this one! It's the only resource I've needed and still use for flexbox. I've got most use cases committed to memory by now, but every now and then I need to visually see what I'm trying to do and quickly reference its code.