This is a Lego Mindstorms compatible ball. Prints in two pieces with a cylinder joint. Glue together (e.g., with ABS juice). Customize if the tolerances aren't right or you want a different size.

I had the most luck printing at 0.20mm in PLA, and gluing with superglue.

This goes well with: http://www.thingiverse.com/thing:2313035

Update: May 12, 2017: Resized for better fit in PLA.
Update: May 14, 2017: Chamfered joint for easier assembly.