The trailer for Hot Lava is something else. It's a five-minute homage to Saturday morning cartoons complete with theme song and an ad break for "the Mount Vile Gauntlet Trial playset". The actual game it's a trailer for is, like that playset, an elaborate obstacle course where—just like we pretended as kids—the floor is lava.
Hot Lava is what Klei have been working on alongside Oxygen is Not Included and new DLC for Don't Starve. In the middle of E3 fever during the week, they quietly launched it into open beta on Steam, meaning that you can play it right now.
They've clarified that this is not part of Steam's Early Access program, however. When games go into Early Access they get a chance to be featured on the front page, go into the recommendations algorithm, become eligible for user review, and generally become a part of the greater Steam ecosystem. Klei don't want this version of Hot Lava to go through all of that. They've had open betas available through their website before, and did something similarly stealthy for Oxygen Not Included. It means Hot Lava will be available on Steam without being visible to people who aren't specifically looking for it.
Apparently the plan is for an Early Access release to follow when Hot Lava's ready for it, but if you want to get in earlier than Early, here's the Steam page.