Ball joints are commonly used in delta-style RepRap designs, most often in the form of rod end bearings (often called "push rods", "rod ends", etc). Although the term "Traxxas rods" is commonly used to refer to rods with bearings on each end, Traxxas is actually a brand which manufactures such rods and the RC cars they are generally used in.
There are several methods of achieving this type of joint.
Traditional ball joints
These are the most common commercially available types. They can be metal, plastic, or a mix of the two.
Description from Deltic3D#V2.0: A few hand drawn sketches of the Mag-Joint Delrin part. The important measurement (marked SECRET!) was determined with a large amount of trial and error. The final value allowed the ball to sit flush in the bottom of the conical shape as well as be firmly held by the outer top hole. On some of the early ones the hole was not deep enough and the ball would sit in the conical but rock around at the top. When the magnets were glued in they were recessed from the conical bottom by a fraction of a millimeter so as to have the strongest attraction to the steel ball but so the ball didn't quite touch the magnets surface.
3D printed joint
Description from Delta-Pi: Hot end platform with a variant of the Kiss hot end and fan cooling