Google sent out development kits to 100 developers around the world.
Earlier today, Google announced Stadia, their bid for the gaming market, and a streaming-only game platform. They managed to announce a game platform without actually announcing many games for it, though they did bring on some developers and publishers on stage as proponents of the upcoming service.
These include id Software and Ubisoft—of course, since we already knew it would. Tequila Softworks, the folks behind RIME, also came on stage to demo a feature (from which it follows that they may end up being a supporter of Stadia in the future). Q-Games, the Japanese based developer, was also on stage.
Google confirmed during its keynote that it sent development kits for Stadia to over 100 developers, and that it is now accepting applications from other developers for dev kits, should they want them, so what we see right now is probably not all of it. And having id and Ubisoft on your side is a good start, if nothing else.