Here are some of my personal favorites:
These were all written by our very own crazed feline, Meowza. He made them as cross-platform as possible. These can be used for Photoshop and GIMP without too much trouble, and even some other web-based editors.
If you're an educator or teacher these are excellent (and free) to use for the purposes of sharing in your class to introduce students to different concepts in image editing. And of course the Phoenix editor itself is now available for your students to use.
They are sorted from beginner through expert, check out pages 2-3 for intermediate effects and 4-5 for some of the more advanced effects.
A lot more to come, including video tutorials and user submissions!]]