I'm a big fan of raspberry pi's, but there are even better, cheaper computers out there.

The ESP-32 is a super low cost (~$10) board that you can build a robot around. I'm experimenting with it in the rosmo-robot and so far it seems pretty good.

It's super handy that it has WIFI enabled, so it allows a roving robot to connect to wifi and then be controlled by your main ROS computer.

Compared with spending about $100 for a full on raspberry pi its pretty competitive.