Hello Worlds! is a game produced as a project of some kind at Washington State University, apparently.
It's a clever little game that, while a simple platformer at a glance, messes with how you have to perceive the world, in a similar yet distinct fashion from Enough Plumbers. (It also has minor points in common with Eversion.)
Whereas Enough Plumbers forces you (especially in its harder levels) to think about coordinating the movements of multiple individuals with the same controls, though, Hello Worlds! forces you to think about how to move one guy through multiple worlds simultaneously.
You might be saying "say what now?"
Well, take a look at this level. You might begin to get the idea.
Here's the trick: Your little guy (which is a gray... bug thing... with three legs) interacts with each object in each environment (which there can be as many as four of at one time) as if each of those environments were overlaying each other.
Which you can see in-game with the "combo" button:
This leads to bizarre in-game visuals, such as floating little bug guys. Once you work on it for a while, though, you get used to it.
However, that doesn't turn the game into a simple platformer once you're used to the setup; no, you also must rearrange the worlds that you travel through (and even "destroy" them) by the use of blue and green doors in order to reach coins and eliminate obstacles.
This level, by the way, is actually a post-"normal game" reward level, from a group of levels that were too complex/counterintuitive to be obstacles from reaching the end.
Or in other cases, too silly.
This is a fun, inventive game that's challenging without being too excruciatingly hard, and I'd strongly recommend checking it out.