Script.aculo.us is a cool library for effects. I collect samples from different places in this page. Enjoy it!
Extra Tip: protoculous is a javascript file which contains Prototype and Scriptaculous together compressed.

Another cool effects (without demo here):

  1. Accordion effect
  2. Auto-scrolling page navigation [new]
  3. Carousel (able to use with Rails)
  4. Crossfader
  5. Data Picker [new]
  6. FrogJS Javascript Gallery [new]
  7. Goofy Boxes [new]
  8. Image Transition Manager [new 2007/11/02]
  9. Lightbox [new]
  10. Lightwindow: Lightbox prototype.js version
  11. Modalbox: like Mac OS [new]
  12. Password strength meter
  13. Prototype window [new]
  14. Scal: calendar component [upgrade in 21/nov/2007]
  15. Slideshow
  16. Slideshow 2
  17. Scroll with slide component
  18. Scroll 2
  19. Shopping cart
  20. Tooltips
  21. Validation [new]
  22. Suggestion? Mail me