30 Seconds of CSSby Atomiks,
Long-scrolling One Pager hosting a curated collection of useful CSS snippets you can understand in 30 seconds or less. You can find the updated non-One-Page site (Oct 2021: the redesigned to a multipage resource you can find here.
This website has unfortunately redesigned or gone offline, so I have removed the link to it. The screenshot below hopefully preserved a bit of the design for you.