GarageGames' InstantAction web site is now open to the public to register and play, though only four games are available at the moment. Multiplayer 3D games like Marble Blast Online, Think Tanks and Screw Jumper runs quite well from the browser, but the best games (e.g. Fallen Empire Legions, Caravel Games' Frog and Mice, Metal Drift, Ace of Aces and Rev) couldn't come fast enough as the ones which can be played right now are rather lacklustre if it wasn't for team play features.

Note: Some people have commented that InstantAction screws up Firefox after the plug-in installation, but it works fine for me. Proceed only with caution.

InstantAction beta web site
InstantAction announcement
Preview: GarageGames' InstantAction
GDC 2013 Will Be Mostly Web Games - Josh Williams, GarageGames
Interview: Caravel Games