Steam now offers a Defense Grid 2 Demo, a sample of this tower defense sequel which was released with little fanfare a few days ago (thanks Frans). Steam doesn't always include demo descriptions, but there is one in this instance since the demo has a separate listing:

This demo includes the first chapter of Defense Grid 2. Defense Grid 2, DG2, is the highly-anticipated sequel to Hidden Path Entertainment’s 2008 Defense Grid: The Awakening. Setting the bar as the definitive tower defense game, Defense Grid 2 introduces new worlds and threats to test your tower placement strategies. With a bold new look, a compelling single-player campaign, and the addition of new game modes, online player-versus-player and multiplayer co-op, every play-through brings fun new opportunities and challenges.