BlenRig
BlenRig is possibly the oldest Auto-Rigger for Blender. It was used in some of the official Blender Open Movies, such as “Caminandes” and “Agent 327” and is available for free.
Unlike other similar addons which emphasize a quick and easy rig setup, BlenRig’s focuses on delivering an extremely feature-rich, film-quality humanoid rig.
The addon attempts to do as much of the rigging and weighting (skinning) work as possible. The full rigging process includes automatic setup of advanced deformers such as Mesh Deform and Lattices.
However, this extensive functionality requires a somewhat lengthy setup. To help aid with that, the newest version of the addon has an Interactive Rigging Assistant Guide built right into the Blender interface which guides you through the rigging process:
However, even with the interactive guide, many users find the learning curve required to master this addon steep.