In robotics, rotacasters or omni wheels are often very useful.
It's possible to buy LEGO-compatible ones from some third parties, here's one random example:
Unfortunately, LEGO has yet to make any themselves, which means if you are looking for a purist solution (because of robotics competition rules for example) you are out of luck.
Or are you? It might be tough to make a small one, but surely it's possible to build one.
What are some good rotacaster / omni wheel designs?