Caption: A Borg sphere breaches the Cube
Caption: The Borg sphere targets Bozeman, Montana
Caption: The Borg sphere heads towards a nebula
A sphere-shaped Borg vessel, usually smaller than the Borg cube. This type of Borg vessel can apparently house itself within a Borg cube, until needed.
In 2373, Picard and the crew of the Enterprise-E followed a Borg sphere back in time, to 21st-Century Earth, in order to prevent the Borg from stopping the first warp test-flight. The Enterprise destroyed the sphere, but not before its Borg crew beamed over to Enterprise.
The U.S.S. Voyager encountered spheres several times in the Delta Quadrant.