Air Safety Foundation: Source of good recurrent and refresher training material. Earn credits towards FAA Wings Program. ASF online courses can be found here. ASF safety publications can be found here.
Here's an unexpected find in this category sent in by a homeschooler doing research on flight. (He found it in, of all places, a car parts site.) The links at the bottom of the page are pretty interesting. (Thanks for the tip, Sam!)