Re: City Blocks - A Different Game to Play With a Copy of Bl
They're all polyominos. From http://mathworld.wolfram.com/Polyomino.html :
A polyomino is a generalization of the domino to a collection of n squares of equal size arranged with coincident sides. Polyominos were originally called "super-dominoes" by Gardner (1957). A polyomino with n squares is known as an n-polyomino or "n-omino."
That page also shows all of the polyominos of 5 or fewer squares, which just happens to be exactly the set of Blokus pieces.