Here's a great video featuring Roller-Walker, a robot built at the Hirose-Fukushima Robotics Lab in Japan. (Thanks, Sam!)

The robot has four legs and a wheel at the end of each. On rough surfaces like grass, its wheels are turned flat so that the hubs can be used like feet. Once it reaches a smooth surface, however, the wheels rotate 90 degrees and it can roll along.

Then Roller-Walker propels itself by moving its legs in a wave-like motion, which resembles the way ice skaters or roller-bladers gain momentum. This graceful motion looks a lot like swimming over the tarmac. I think the Roller-Walker also does well not to lose balance when standing on three wheels and readying its fourth.

New Scientist Technology Blog: Roller-walker 'swims' on four wheels