Singaporean artist Lee Wen first came up with the idea of a circular ping pong table in 1998, when he presented the prototype in the form of a street performance in Melbourne, Australia. In 2012 the project was re-enacted at an exhibition at the Singapore Art Museum with an Olympic standard length large version in the front the museum, and a smaller one inside. As evidenced by one of the images about the testing of the ping pong roundtable, this device can be used for some intense work out as well, especially by the person who plays (and runs) on the outer ring.
By the way, there exists a small ping pong roundtable as well, which seems to be a perfect alternative for the traditional 2-player and 4 player variants of the game. And you can actually buy this one, here.