Just a concept I recently tried out and thought was useful... By making the Command Center provide Zerg Supply and removing the SCV's ability to produce Command Centers (instead, Supply Depots are resource depots and produce SCVs) you can limit the number of Zerg Supply the player gets, because they cannot obtain more.
How is this useful?
Well, I made the Command Center able to produce hero units that cost 1 Zerg Supply (And the CC provides 1): thus limiting the player to only have one hero unit at a time.
Nothing really revolutionary, just something I thought was interesting.
Of course, like all things dealing with supply, there is the issue of Mind Control. To have it work flawlessly, you have to set aside one Supply for this purpose.
Has anyone found an easier way to do this? If so, tell me!
Other things:
1. Units can require more than one race's supply.
2. If a unit's availability flag is Independent, Neutral or Factory, it will display the error "Not enough Vespene Gas" when you try to build it, no matter how much gas you have.
3. Having a unit provide Independent Supply adds the words "Disabled 0" in red text underneath "Supply Used".
4. Units cannot provide more than one supply.
5. Setting a unit to provide Independent supply does not work.