We kinda knew this was in the works, and I think we're going to see Half-Life 3 rolled out with it. But, during a visit from 4chan.org's /v/ board for his birthday, Valve boss Gabe Newell received some presents from some randoms from the Internet.
He had to pay $2.50 to unlock a real-life Mann Co. crate that included a TF2 Soldier hat inside, and he hung around to talk to fans of his, and his games, that came from all across the United States. The first question asked, and paraphrased from the above video, "Is Valve working on a new Source engine?"
Newell replied with "We've been working on new engine stuff for a while", adding "we've just been waiting for a game to roll it out with". Someone then follows that question up with "is it more than just an extension to Source? Like, is it an entirely new engine?" Newell, the hero that he is, replies with "yeah."
We all know Source Engine 2 would currently be in development, but I think we're going to see Half-Life 3 as the game to show it off. I'd expect sequels to Left 4 Dead, Team Fortress and more afterward, too.