If you have a motor that doesn't need reversing, it can be powered by a transistor setup as shown here. If you do need to reverse it, an h-bridge is called for. The h-bridge chip doesn't have to be on a shield, nor even on a factory built board, and can be made from scratch using a chip like a 298 on a breadboard as shown in this tutorial.
H-bridge chips like the 298 (which by the way, is not the only or even the best h-bridge chip) drive 2 motors, and so shields / boards using them allow you to hook up 2 motors very easily.