The free Portal 2 DLC announced by Valve in April will be released in mid-September. According to Valve's own Doug Lombardi, "That's still technically summer." And he's right! Summer officially ends on September 22, so we can hopefully expect the downloadable content by then.
But let's be serious, this is Valve we're talking about, so it's not surprising that the DLC has taken a very long time to see release. "DLC #1" will offer new test chambers, leaderboards, challenge mode for single and multiplayer modes, and more. It will be absolutely free of charge for all versions of the game, including Playstation 3, PC, and Xbox 360.