Quite possibly the best SC2 mod ever: SpeedCraft

Crazy mod. Everything is free and has an instant build time. It's as crazy as it sounds.

Vid:

Skip to 40m 28s for the mod demo.

ohh i love this, it existed in sc1 as well
the most popular versions of it combined it with a few other things
my personal fave was a combo with survival and regular melee. the map was a normal 8 person ffa where one corner of the map was occupied by a computer zerg.
this was sc1 so any kinda custom melee ai opponent wasnt really a possibility so the computer just camped in its base for the entire game using custom units that were enhanced to be ridiculously powerful (but bc of how sc1 worked the "buff up" applied to all zerg units so you couldnt play zerg on maps like this). there were occasional triggered actions that made it less monotonous
if you still hadnt broken the zerg base after a while (it was sorta random) a bunch of super mutas would spawn and roll everyone

the game was pretty fun. it woulda been really fun but it basically summarized to "how many nukes can i make"
it was also funner without the game mode joe is talking about so im not sure why i posted about this at all but i did so here we are