Luminous Curves: I can’t tell you how much I have learned from the guys at Luminous-landscape. This is a very basic tutorial, but they have loads of other information there, so I am happy to pass on this link.
Using Curves: This is a bit more thorough than my article with loads of great information, including some precautions to avoid.
Earthbound Light: Great information on curves for people using Photoshop Elements, including information on the Smart Curves download and Curvemeister add-ons.
Handy little things: It is possible to save your favorite curves and apply them as needed to different images. This article tells how to use them and gives you a link to the author’s favorites.
Beyond the basics: Great article on going beyond the basics of curves to create some really interesting effects. Also includes some little known secrets to working with the dialogue box.
Good starter tutorial on curves. Shows you a way to place markers along the curve that can be helpful.
Curves Essentials: Sorry I can’t embed these videos for you, but they are worth the effort. There are four videos on curves; a setup video, followed by three more that go into further detail (videos 14 through 17.) Included is Dave Cross’s nifty trick for finding middle gray.