The Sirui LE-60 Leveling Base fits between a tripod and video or ball head, and lets you quickly adjust and level the head independently from the tripod.
It can be used with any tripod or monopod with a 3/8″ stud.
The LE-60 has a 2.4″ platform with an off-set bubble level that can swivel 15° in any direction, and is rated to hold camera systems weighing up to 15kgs. The top platform is compatible with heads with 3/8″ or 1/4″ mounts.