1. Coffee Steam Effect
This tutorial teaches you how to create a coffee steam effect using curvy lines and tweens. I have to say, this tutorial is quite long and very tedious to follow. But the author does provide a download link to the source file with the finished effect at the end of the tutorial.
2. Underwater Effect
This underwater effect has a very nice and subtle feel to it. It makes use of a combination of masks, gradients, AS3 and the perlinNoise() method. The effect was placed over an illustration, but you can try modifying the technique to make it suitable for a photo-realistic underwater image as well.
3. Water Ripple Effect
This is another water type effect that produces a nice, subtle water surface ripple. However, it's written in AS2. But nonetheless, it's one of the nicest effects of this type that I've seen. It uses the perlinNoise() method as well. No step-by-step guide on this one, but the source files are available for download.
4. Lightning Effect
For this one, the author uses AS3 to create these really cool lightning / thunderbolt / electric discharge effects. The code is a little complex, and the author doesn't really explain it much, but still, the effects are pretty awesome.
5. Waterfall Effect
This one uses a particle system to create a stunning waterfall effect. The author created a custom AS3 Waterfall class, which allows you to change the appearance and style of the waterfall by changing a few parameters. You can even set the waterfall's background to transparent and then place it over an image. This effect is quite beautiful.