Why, a bamboo-framed mountain bike, of course!
I can't begin to say how cool this is. From my memory of when I knew heaps about mountain biking: The properties you want in a frame tubing are resistance to bending (excellent, they make scaffolding out of this stuff in Asia), resistance to splintering under hard loads (well...), a good strength-to-weight ratio (yes), a good degree of shock absorbancy (I imagines o) and, ideally, cheap (bingo!). I love the very idea of this, let alone the way that this bike has been executed. And you can get the frame lugs made out of hemp fibre instead of carbon fibre! Which begs the question: Can you make an entire frame out of hemp fibre?