Breakthrough is a free and open-source game which has one of the simplest rule sets among board games.
Breakthrough develops quickly because pieces have no choice but to move forward. Draws are mathematically impossible, which is a rare feature for move-based games.
Breakthrough's strategy is novel and sophisticated. A piece can 'skirt' around an opponent piece by moving directly in front of it, where it cannot be captured, then moving diagonally around it.
Consequently, a single piece cannot by itself prevent an opponent piece from 'breaking through' the ranks.
The defense consists of arranging pieces in tandem so that the opponent cannot skirt around them. Basic offense consists of attacking opponent pieces that guard multiple attack routes, which may open up a route.
Breakthrough is intuitive and immediately playable, but very difficult to master.