Rules
Divide the grid into rectangles such that each rectangle contains exactly one number and such that this number equals the area (i.e. the number of cells) of the rectangle.
Example
Here is a grid of Shikaku and its solution.
Solve the example online here.
History of Shikaku
According to the WPC unofficial wiki, Shikaku is a puzzle first published in 1989 in Nikoli. It was invented by Yoshinao Anpuku.