I've been playing since day one and I'm always trying to find out new things for maps, matchups, units, etc. Useful or not, it's good to know what your game is capable of. It's more fun innovating techniques than perfecting builds and timings to me (imo, these things are the basis for builds and timings in the first place so it's more important), but feel free to post stuff like that too.
I'll keep posting stuff here as I find it, and I hope you guys do too. Please keep the "WTF OMGz!" to a minimum, as they will clutter the thread.
-Vikings suck ass ground or air. But the one thing they are good at is hit and run versus BC, (BC can yamato though, so watch out).
-you can toggle unit health bars into 3 modes in the options, alternatively you can press alt any time during a game to hide/show health bars.
-Reapers/Stalkers can hit and run versus slow lings and zeals early game.
(by hit and run, I mean they won't get hit at all with proper micro and will kill the enemy melee)
- A hatchery's vision range when it is morphing is very small, you can place a bunker 1 or 2 spaces away and not be seen.
-On Scrapyard Stalkers can blink from main to middle island from either base. (you need vision though).
-All units can be patrolled a very small distance and they spin around all crazy (looks funny).
-Obvious, but Terran can fast expand to maps with islands and rocked in bases. (Kulas Ravine, Scrapyard, Desert Oasis, Steppes of War, LT)
-Air units bank around when they turn (looks cool, but this is arguably bad for the game), if you keep a rhythm of clicking around in a circle, you can get almost all air units completely sideways/vertical.
-You can set buildings rallies in any direction, units will pop out on the respective side of the building.
-The buildings in SC2 all fit to a grid, and you can wall with nearly every building. Walls block everything from small units (like lings) and up.
-Taking the last two into consideration, a teching technique I found for protoss, was to make your base in such a way to trap a stalker and use it like a canon/bunker as you tech up toward whatever you are going for. http://tinypic.com/r/w15qo9/6
PS- I'm sure people probably think it's dumb of me to reveal secrets/techniques this early into the game, but I think that the better everyone is, and on an even playing field, the funner the game is.