English Country Tune

While increpare games only has three products listed on Steam, the person behind the name, Stephen Lavelle is (in)famous for his creativity and his volume of output. According to Wikipedia, he has released more than 400 games, mostly freeware - and three commercial games; English country tune is the first of the three.
Apparently, Stephen has heard about the idea of gentle introduction to difficult puzzles, and the first three levels look like a stylish but slightly weird (the protagonist is a flat square) Sokoban-clone. Then maybe he thought "let's not overdo gentle", and level 4 hits you with two of the game's landmark features - the 3D movement and the relative gravity.
Regarding the movement, the game seemingly effortlessly translates the two axes (up/down and left/right) into the three "real" axes - such a solution should have no right to work, but it actually does, and very well. It's possible to get yourself confused and misnavigate, but due to the unlimited undo and the (almost) complete lack of timing needed in the game, it's never an issue.
The relative gravity is... weird. Your first creature (crate in Sokoban), the Larva is not affected by real gravity, but when you push it, it will behave as if it was, and the direction of the gravity is relative to the direction of the push... I mean, who can come up with such an idea? :D As you're in real 3D, this means a Larva that hopelessly falls out of the world when pushed in one direction may move easily to safety when pushed from another. I have to admit, even after finishing the game, I can't say I 100% wrapped my mind around the idea, but the puzzle design implications are just awesome.
Moving forward, the game introduces more creatures and more mechanics, and as you could expect, it's rich in "a-ha" moments - although in English country tune, these are more like "wtf" moments, "holy s**t this is brilliant" moments and "oh my god this guy is insane" moments :D
As much as I love the game for its cool visual design and its (mostly) amazing puzzles, I can't say it's without flaws. While the controls are brilliant and the automatic camera usually does a great job, it sometimes doesn't, and you cannot freely choose your viewpoint - you can yank the camera around, but it always returns to its default position. Also, some of the puzzles feel a bit stale, and there are a few which feel downright silly (I took a particular dislike to the Resonator ones, but ymmv).
However, faults or not, English country tune is a very ambitious game which is almost fully successful in reaching its rather high targets. The creator is a puzzle making genius, but it's also obvious from the game that he spent a lot of time tuning and refining the puzzles, and most of them present you with fresh and unique challenges. It's a hard game, rather unforgiving at times, you need to dedicate time and attention to it - but when you do, it's one of the most rewarding experience in the already pretty rich indie puzzle scene. Highly recommended.

Coming to this after enjoying (but not completing) Steven's Sausage Roll, I was very happy. The levels here are also hard, but generally speaking I didn't find them as difficult as the ones in SSR. At the very least, I was able to complete all of them here, eventually. Much like SSR, the mechanics are generally fairly straightforward, but thoroughly explored in order to truly test your understanding of the mechanic and logical deduction skills. So this is a great game if you really like having to think things through, figure out how mechanics interact, and generally figure out what's required of you. It's pure expert-level 3D puzzle design.

The puzzle 'Half Sided' will melt your mind.

This is more than 3D Sokoban! It's 3D Sokoban with relative gravity! Super awesome mechanics and level design. It's pretty similar to a game inside the arcade hub Fancade from Martin Magni, called This Side Up by LukaszM. I myself made some difficult custom levels for that game! Hey, if you read this I'd really appreciate if you would check them out and leave a comment here! =]

This Side Up by LukaszM:
https://play.fancade.com/610FC0AF79952EDE

Hard Side Up by Linkblade:
https://play.fancade.com/628762537C982F25

Edit: I only touched the tip of the iceberg: the twisted sokoban is actually just one of many different tile-based 3D puzzle games waiting for you and further for each their are combinations with the other games! Very recommended! All over not too difficult as the unlimited undo button is a big relieve.

Edit2; I take back "not too difficult" and replace it with "gets really difficult and mindboggling". Half-Sided level set gives you one mechanic and lets you choose everything else in a level editor, how the 3D blocks are placed and where you start. This mode has so many possible options that it gets really hard to find the solution.