Recs.
Updated
SpaceChem is a 2D optimization-puzzle game themed with pseudo-chemistry in which you build machines that modify atoms or molecules with bonders and waldos (these pick atoms up) to make other molecules. Then, the game progresses by building reactors and introducing more concepts.
SpecsUpdate
Pros
Pro Endless gameplay through user created solutions
An individual players choices of components, arrangements or factories used will result in completely different experiences and problems/puzzles when compared to another users experience. A truly individualized experience that expands the games replayability.
Pro Allows for short or long play sessions
The gameplay allows for a player to come in and solve a single puzzle in a short amount of time or to sit and tinker with a puzzle for hours trying to devise the perfect solution and contraption. It all comes down to what one want to put into the game but the free form of the puzzle solving is what makes it great for anyones playstyle be it short or long.