Advanced Skeleton has been used to rig creatures, props, vehicles and just about anything since 1999. It is free to download but if you want to use it for a commercial project you will have to buy a license.
The Setup Machine enables Maya users to easily and quickly rig a wide variety of CG characters with just a few clicks. Generate a flexible, modern character rig in minutes saving you days or weeks of work
Rigging Dojo is a website that helps artists overcome the obstacles of computer animation production by focusing on teaching the art and science of character rigging. They have tutorials for both rigging and animation.
Mixamo Auto-Rigger is a free online tool that allows you to upload any 3D human model with two arms and two legs and get it rigged with a full skeleton and skinning weights.