If you select "Join Custom Game" and search for a map - for example Xel'Naga Caverns - you can join the game and play against a seemingly random player who had created the open-to-public custom game.
But how does battle.net select the game? Is MMR considered? Is it purely random? First come, first serve?
Similarly, is it possible to spectate open-to-public games on the standard maps? I'm thinking this could be an interesting alternative to watching live streams.
Edit: As answered by @Resorath, it seems to be first-come-first-serve. Do people find this feature useful / fun then? I would think it's pointless to play against opponents that have no relation to your skill level. I guess it could be interesting for practicing a risky build order without damaging your ladder ranking, if you don't have a practice partner.