Post some of your fav here:
Here's one of mine for Protoss.. Scout+Gas.
With a slightly reduced mineral cost of an Assimilator, with 150 minerals you can cover both gas patches on an enemy base while scouting it with a probe. Any diversion of their workers to destory will cost them valuable economy (likely more than the 150 it cost you). And once fully warped in they have 900hp to chew through. It will definetely set them back on units requiring any gas... and if you want you can go right into a cheese cannon rush while they are distracted with destroying the Assims. I think key is to do it as early as possible. If they don't destory both immediately you also have a temporary scout in the players base that allow you to observe build order and counter appropriately.