Blokus is a strategy math game for 2-4 players that develops and builds spatial reasoning skills. I also use it to explore the concepts of area and perimeter as well as symmetry. In addition, it introduces transformations including reflections, rotations, and translations. Blokus was first made by a French designer, Bernard Tavitian, who had not only a master’s degree in mathematics, but an engineering degree and also a doctorate in biophysics.
There are four sets of colored game pieces. Each of the 21 sets contains a different arrangements of 1-5 squares. In fact, as Math For Love explains, the 21 pieces represent all of the possible arrangements of 1-5 squares. The pieces with 4 squares are the same pieces used in Tetris, as most adults immediately recognize!
After playing Blokus it is fun to explore pentominoes. 
Basic Blokus Game Play
To play the game, two-four players take turns placing one of their colored tiles on the 20 by 20 grid gameboard, starting at their corner. Each piece played must touch another piece of the same color, but only at the corner- the edges can’t line up at all. The objective is to place as many of your pieces as possible while blocking the other players. Game play takes about 20-30 minutes.
Other Ways To Play
The first time I introduce the game to children I have them work together to place all the pieces on the board, fitting them together however they want to. Later I will see many individual kids using the game board in this way just for fun. Little do they know they are building their spatial reasoning and geometric skills!
On Sarah’s Stockpile of Math Enrichment Resources, the author explains two adults exploring the question of whether they could collaborate to fit all the pieces on the gameboard following the rules of the game that pieces touch other pieces of the same color but only at the corner. This would be very fun to explore with kids! (If you’re really into the mathematical possibilities, see Blokus Discoveries, which used a computer program to find them!)
You can also use the pieces on the board to create symmetrical designs.
Blokus Online
You can play an online version of Blokus at Funky Potato.
Competitive play with players from around the world (free account required) is offered at Pentolla.
You can also download a Blokus App on iTunes for $2.79
Board Game Variations
Amazon sells an exclusive Blokus Deluxe edition with a larger board. Most reviewers report the larger board is not worth the extra cost.
Blokus Trigon, also an Amazon exclusive, replaces the square board with a hexagon. The game pieces are made up of triangles rather than squares.
Blokus Duo is made for only two players. The number of pieces per player is the same (21) but it uses black and white pieces instead of colored pieces.