Boomerangs are weapons carried around by the Boomerang Bros.. When thrown, a Boomerang will eventually reverse its direction and fly back to the Boomerang Bro that threw it, attempting to injure the player along its path.
In Super Mario Advance 4: Super Mario Bros. 3, there is a special variety of blue Boomerangs that Mario can find and use, although they are very rare. The only level where Mario can find one is in the World-e level Slidin' the Slopes. Near the end is a Boomerang Bro with a blue Boomerang. If Mario defeats him, he will drop his Boomerang, which Mario can pick up and throw. If Mario fails to catch his own thrown Boomerang, he will lose it. However, since it comes at the end of the level, it will be of little use to Mario, as it disappears upon completion, much like Kuribo's Shoe.
There was a japanese e-reader card which lets you scan them into the items list. Boomerangs can also collect coins