Levelhead is part-platformer, part-level builder, and has just entered Early Access. It's the next game from Crashlands dev Butterscotch Shenanigans (one of my favorite developer names, by the way), and as you run through its platforming campaign you'll unlock new items for the level editor, in which you'll build levels and share them with the world.
Butterscotch Shenanigans says a full release is planned within the next six months to a year: during Early Access it plans to add more level-building options—there are currently more than 100 "creative elements" in three biomes—as well as more levels to the campaign, a better-integrated story, and new algorithms to help you discover the best player-made levels.
In the campaign, you deliver packages for the Bureau of Shipping, making your way through a series of dangerous levels while using power-ups to fly and warp through walls. You can play through the campaign solo or in couch co-op with up to three other players.
For level sharing, you'll be able to subscribe to other creators to keep abreast of their latest work, and browse levels by tags. This is where the meat of the game will be: I can see myself blowing through the campaign once and then concentrating on user-made levels.
If you want in, it costs $20/£15.50 on Steam, and Butterscotch Shenanigans plans to "gradually increase" the price as it adds new features.