StarCraft II: Heart of the Swarm will introduce a few new units to game's tried and tested roster. Normally you'd have to wait for the actual game/expansion for this, but why wait when there are mods?
An enterprising soul by the name of XenoX101 is working on a mod that, using both the details and images released so far of the new units, drops them into the existing StarCraft II multiplayer, making them playable on a custom map.
They won't look or play exactly the same as the finished product, but hey, they'll probably be close. Think of it as a beta!