For a wheelchair that can go anywhere, look no furthur than the Tank Chair. This custom off-road chair "conquers streams, mud, snow, sand, gravel - allowing you to get back to nature - and can also climb up and down stairs". It features motors built by NPC Robotics, no word yet on pricing. Video clips here.
For every 10 chairs sold, TankChair is going to donate one to a fire department in a rural area so that they can give it away to someone who would not normally be able to afford one.