LEGO Success.

Lego, officially trademarked LEGO, is a line of construction toys manufactured by the Lego Group. colorful interlocking plastic bricks and an accompanying array of gears, minifigures and various other parts. Lego bricks can be assembled and connected in many ways, to construct such objects as vehicles, buildings and even working robots. Anything constructed can then be taken apart again, and the pieces used to make other objects.

Bohus from the retro thing-blog writes:

There are lots of toy companies who’d like to build on Lego’s success. Lego snapped together an empire thanks to their compatible system of interlocking bricks. Lego pieces from 40 years ago mate with today’s bricks perfectly. Over the years, many companies have tried to duplicate Lego’s achievement, but were unable to create blocks that could mingle with the big brand.