A fun way to use custom value.

In my map i needed players to own a unit but not be able to order it. So how i did was that i stored the player id of the owner in the custom values field of a unit. All the units were own by player 12 so the player couldnt move them. Then when a player got control of the unit i changed the team colour of that unit.
This way you just check the custom value field of a unit to see who owns it. Its very practical.

Indeed there are many uses. In my Worms map which hasn't become popular, YET, because I havn't had a chance to distribute it on bnet yet ;)
One of my items, Death Spike, makes a unit that tracks down a random player and kills them. I used the custom value so that the death spikes could keep track of which player they were after, so if the player used invulnerability or died before the death spike reached them, the trigger could see that they were chasing them and remove them so they dont just stand there with nothing to kill :P