Floor traps to make a tile floor.
Wall traps/doors to make walls, or cover walls with another pattern.
Use of all the traps to create interesting block shapes.
Floor missile traps shoot like goliaths missile, but hit ground units instead.
Abilities hit traps, like psionic storm, ensnare, plague, stasis, recall, blind, lockdown, etc...
And... uhm... traps also are detectors!
Thats all I know about traps.
Also traps take terrain vehicle upgrade, but thats not really an effect.
How about masses of cloaked units? That can be a good visual effect.
Burrowing dust can be used as an effect.
Those protoss shield hit sparks are another one.
Slowing down a ground unit... is that considered a visual effect?
Nukes/Interceptors flying around with junk yard dog.
Recall hits, for teleporting units in, or spell effects or whatever.
Turning units to face different directions or spin around. If you make a movie or something you may want your characters to face a certain direction.
Change a unit's hit points rapidly to make a glowing or flashing effect when you select them.
Move units from under and arbiter rather than create them so they fade in from cloaked.
Create a unit from the same spot antoher one was, to make it look like it changed. (Maybe with another effect to cover-up)
Use sprites intead of actual units to fool people into thinking a place is blocked off when it actually isn't... hehehehe
Rescued units retain their original color, could be used for anything really.
(Can you give a unit to the player it already belongs to to re-set it's color?)
My very favorite ability visual, EMP shockwave, I don't know of any way to get the computer to use it. If someone does, tell me.
Create units under a transport and play the unloading sound to fool people into thinking the transport had a ton of units, like a clown car!
My units-shooting-across-the-map thing dosent work anymore... that was a great one though.
And don't forget you can use text or ascii art for effect too!
This is bigger than I planned. There has got to be an error somewhere. Find it! Ha!