Meet GoBrix: a cool kit that lets you motorize your brick creations. GoBrix RC brick motors are compatible with LEGO, Mega Bloks, KRE-O, and other brands. You get a two-motor or single motor add-on set to choose from. These motors run on AAA batteries.
GoBrix comes with everything you need to bring your LEGO creations to life. ThinkGeek has these for under $20.