This thread provides links to useful threads that have been stickied in the past and continue to be useful. Before reading through the archived threads, make sure to check out the main CSS sticky:

The CSS FAQ — This super thread provides solutions to the most common CSS problems, and should be required reading for everyone!

Please also check out the Forum Posting Basics thread, which gives important tips on posting code samples.

The Mighty CSS Quiz archive — Take your CSS skills to the next level by checking out these amazing, groundbreaking quizzes by CSS guru Paul O'Brien.

Further CSS Challenges — A recent series by Michael Morris:
CSS Challenge #1: Teaching a friend CSS - Lessons log.
CSS Challenge #2 - Checkerboard.
CSS Challenge #3 - Things are about to get dicey. CSS positioning.
CSS Challenge #4 - Drop down menus
CSS Challenge #5 - Flip A Card.

Tutorials and Documentation — A developing effort to provide documentation on a range of topics.

Why Switch from Table to CSS layout? — Most people have made the switch now, but still, a useful guide to the reasons behind the change.

CSS Tips and Tricks — An oldie, but a goodie, with plenty still to discover.