The Atomlab General Issue frame is hard to beat for the money. It's a versatile jump/ dirt riding beast made from straight-gauge, heat-treated chromo steel. It has vertical dropouts, it's compatible with 24 or 26in wheels, and you can run either V or disc brakes.
This rig is pretty cheap but it rides just as well as many of its more expensive peers. The 22.5in top tube is very roomy, it offers a great feel for taller riders and the head angle is steep enough to keep the steering quick but not overly so, which means that new riders won't be intimidated by its BMX-style geometry. The 16in chainstays are also on the money for a dirt jumping bike.