SpaceChem
A game of manufacturing molecules on automated assembly lines.
A game released in early 2011, spacechem is a puzzle-based 2d game loosely based on chemistry. In gives you challenges, each rising in complexity, which generally means transforming input materials to output materials. Materials are atoms or molecules, each no larger than a 4 by 4 grid with one atom per square, and the means of manipulating them are two "waldo", which are assembly lines that can be programmed to do tasks by placing command symbols in their paths. Symbols can range from the simple grab/drop/rotate atom, to others that activate hardware such as bonders/debonders, fusion and fission lasers, quantum tunnels, etc.
Yes. It's about this complex.
While a good portion of missions take place in a single reactor, there are cases where you have to design a reactor chain, in which case you usually have to divide the task you are given into smaller sub-tasks, and design the reactors for them.
But not all solutions are treated equally. Sure, they do solve the task, but each symbol placed, and each "cycle", the time in which a waldo travels one square (horizontally or vertically), is logged. Faster solutions and those that use less symbols are aways considered better solutions. Speed is also of the essence in certain "boss" missions, in which you have to accomplish a task before the boss destroys you. This can mean creating a fuel for a rocket to blast a giant...
I think you get it.
The game also tracks solution records, and compares your solution to the statistics found online. In this way you can see whether there are better solutions, and the portion of the playerbase that solved it that way.
But aside from offering challenges for the brain, it also has an intriguing story, which is told in a book/comic book format, with new pages unlocked after certain missions.
For those who are done with the "campaign" and want more, there are also online challenges that can be accessed ingame. You can also contribute and design challenges for other people, if you are of that particular mindset.
A word of warning though: It's incredibly addictive. I remember spending 15 hours almost straight on one challenge... and I dreamed of waldos for days. It also becomes difficults after a point.