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?
I have copied a design by Isogawa Yoshihito
I prefer it over the Lambert Varias design as the wheels are positioned more radially
More pictures in this EuroBricks thread: https://www.eurobricks.com/forum/index.php?/forums/topic/76297-isogawayoshihitos-lego-technic-omni-wheel-crab/
Here's a design I've been using, based on this model by Lambert Varias:
It's not a perfect circle, but it is relatively small all things considered:
It's pretty simple, but here's a partial disassembled version to better show what goes into it:
Parts shown:
