Valve has announced that Defense of the Ancients 2 (DOTA 2) will release for both PC and Mac next year.
According to Valve, DOTA 2 will be "a massive sequel for one of the most played games in the world". It will run Valve's Source engine and will include Steamworks achievements. There will be no single-player component offered outside of training missions, although it's understood that the AI will take over in the event that a player drops out of a match.
The original Defense of the Ancients was a fan-made map variant from the game Warcraft IIII: Reign of Chaos that was not officially sanctioned or supported by developer/publisher Blizzard Entertainment. It became so popular that it developed its own cult following quite independent to that of Warcraft III itself. It's even become a staple in tournaments at the likes of BlizzCon and the Asian World Cyber Games.