I've run some stuff that could be considered "gimmicky", since with Pokemon being all about prediction being able to do something they can't predict can win games. Nothing truly noteworthy, but fun nontheless.
I guess my favorite was HP Fire on my Starmie. I was in a hiatus when Platinum released, and when I came back I discovered that Scizor tore my team a new asshole. Since everything on my team had a role, and I was a bit attached to it, I removed Psychic, which I rarely used anyway in Steel-heavy metagame, and added HP Fire. It racked up a lot of surprise kills. People usually tried to get a SD set up with Scizor, and would get roasted by HP Fire. Not so effective against Banded sets, so I might have to scout the Scizor first, but it destroyed SD sets and helped my team deal with it better short term while I thought of a more permenant solution, which ended up being Zapdos.
I also ran a pure status Smeargle lead. It ran max speed with a Scarf, packing Trick, Spore, Will-o-Wisp, and Rapid Spin (filler). Totally shut down if it got Taunted after Tricking, but it could give me total momentum at almost any time I pleased. My team didn't care about SR too much, so I could Trick Stealth Rocking leads, and then Spore on the next turn. If they stayed in, then their lead was put to sleep, which was great against Explosion leads so they can't use a fast Explosion on me later. If they switched, something else got disabled. Then whatever they switched in got burned. It could Rapid Spin if it got the chance, which while not necessary usually, was helpful, and was great death fodder if really necessary and he had done his job.