Mugenbine are toy robots whose parts can be configured to create original robots of different shapes and sizes. They are really popular among boys right now.
Shape-shifting robots from TV shows like the Super Sentai series (known abroad as The Power Rangers) have always been hits among Japanese boys. The reason for unprecedented popularity of Mugenbine is that they allow children to reconfigure the shape of the robots in any number of ways. That's why the toys are called Mugenbine: mugen is Japanese for "infinite," while bine comes from the English word "combine." And since the robots can be reconfigured over and over into bigger size and more impressive shapes, you could also say that they are an infinite source of fun. The toys are sold together with candy at supermarkets and other stores.