No, I'm not talking about those Space Balls.
NASA will be landing the truck-sized Mars Research Laboratory on Mars in Fall 2009. If the Swedes have their way, the gigantic rover will be accompanied by a number of ball-shaped probes that will roll around the larger probe. The idea is that the balls can go places the rover can't, and that their round shape will prevent them from getting stuck.
It all reminds me of the probes that used to orbit seaQuest DSV (yes, I used to watch seaQuest). Sure, we're talking sand instead of water, but it's the same basic idea, and very cool either way.