Blizzard-made StarCraft II mods Aiur Chef, Left 2 Die and StarJeweled are now available for beta testing.
Aiur Chef is a cooking game. Each round you must explore the map, fetching ingredients to complete a dish before your opponents can. It's a bit like Ready Steady Cook but with killing.
StarJeweled, a puzzle-strategy mod, involves playing Bejeweled-like grids to make energy to build units and stomp on your opponent.
Left 2 Die sounds like the most interesting of the three. Like Left 4 Dead, this involves fending off a never ending 'infested' zerg with a friend. By night you'll fight, but by day you'll build; the action subsides and units can be trained and defences bolstered. Watch out for the Kaboomer and Stank enemies.
If you like Left 2 Die but think it's a bit easy,you can always try Night 2 Die. This takes place during an everlasting night, when wave upon unrelenting wave of infested attack.
To test these out, and why wouldn't you, go to the Multiplayer tab in StarCraft II and look for the custom games window.
There's more blurb on Battle.net.