Fantasy Crocodile Rig is a free Maya rig that has a super appealing design.
Akeytsu is a piece of 3D animation software and is a lighter, faster and cheaper alternative to Maya. It also has a free educational version.
Rocket Girl Rig is a free Maya rig from the VMComix animation blog.
Jasmine Rig is a free rig for Maya that has a load of great features!
Morgan Loomis has a bunch of great animation tools and scripts for Maya on his website free to download. You can download the full animation shelf or get them individually!
Animation Paper is a popular free software for hand-drawn animation created by Niels Krogh Mortensen