ComPressure, a puzzle game about designing increasingly complex computation units powered by high pressure steam is now technically free, as the demo is the full game.
This way more people can try the game, and keep playing as long as they want. If people like it enough, they can then buy it. Exactly what a demo is for and quite an interesting decision, and similar to what the developer of Rings of Saturn did (although they limited saving/loading).
Why though? The developer mentioned hitting over 1,000 purchases and they're doing it as they're "now mostly interested in reaching as many potential engineers as possible". In a more expansive comment on Reddit, the developer explained that instead of making it free with a paid supporter DLC they went with this way due to the type of game it is and how they didn't want 100s of review of people not understanding it. That, plus allowing "players from poor families to play the game, learn electronics/computer engineering and get a decent job" while also keeping out the free game crowd who "are not interested in that and just want the +1".
Certainly can't argue against a developer testing a way to open up their game. Steam doesn't make things too easy for developers like this, since there's no donations features.
You can play ComPressure on Steam.