Here are a few special effects I have picked up over the years.
Water(Top Down)-Make a texture which looks like water. Create a backdrop out of it. Now take that same texture and trim it down 5-6 pixils on all sides. Create 2 to 3 active objects and set their transparencies to about 100(100 out of 128!). Set the active objects as bouncing balls and have them move at a speed of 4. Lastly, set them to look at a point located directly in the center of the original water every 18 (of a second). If you do this correctly the water will move back and forth in random directions in your body of water and it will go from a blurry to clear look.
Fire-You will need a fire animation, several is better. Set a few transparent and have them shake by using a ball movement and having them look at a point every 5(of a second)
Bullet impact- Bullets make bullet holes. Why not try to put some in! Have the bullet, upon being destroyed place an active object in front of it which will paste itself into the backround and destroy itself. This object will be a bullet hole. This effect works even better when you use fastloops to create your bullet.
Color mix-Make two objects, one will be blue, and the other yellow. Both objects will have a horizontal line with pixils followed by a horizontal line without pixils without pixils(transparency). Now place the objects over each other except make the gaps filled in each object. The colors should mix if this is done correctly. Some colors do not work very well. This could be useful for maybe a projectile, or a door...or something cool like that.
Motion-Lets say you have a penny falling. When you see something moving very quickly it often becomes fuzzy, or blurry. Take your penny(or object, whatever that may be). Now create 3 identicles of the penny except each one will be more transparent than the next. You must setup a way so that when the penny(or your object) is going fast, that the transparent pennys will fall behind your objects from most transparent to least transparent. I made it so every 20 of a second I added 1 to alterable value A. Then, ALWAYS I would add 1 to Y of the penny. Next I took the transparencies, and has them set their Y's to the Alterable value of A of the Penny and divide it by 2 for the first penny, 3 for the second, and 6 for the third. Then you take the result and add it to the tranparencies.
The transparencies will slide behind the penny in a very cool looking effect(see example)