The Ray Rig is a free Maya rig useful for dialogue scenes and performance offered by CGTarian
Ray Rig (FREE)
Morpheus Rig v1.0 (FREE)
The Morpheus Rig is a free Maya rig created by Josh Burton available to download from his personal website
Minion Rig (FREE)
The Minion Rig is a free Maya rig based on the popular Minion characters from ‘Despicable Me’
Graph Editor Redux
Graph Editor Redux is a redesign of Maya’s curve editor toolset. Not only does it reorganize the classic graph editor functions, it provides new tools that many animators have found extremely helpful in massaging animation performances.
CG Monks Toolbox
Cg Monks Toolbox is a free toolset for Maya that includes a large selection of handy animation and rigging tools
Morgan Loomis Tools
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!
Tiger Rig (FREE)
The Tiger Rig is a free rig compatible with Maya 2015+
Josh Sobel Rigs (FREE+)
Josh Sobel has some great affordable rigs (his Kayla rig is especially great). You can also get his Bonnie rig for free!
Longwinter Rigs ($10/month)
Longwinter Studios are offering access to their whole rig collection (30+) for only $10 a month. Not too shabby!
Animator's Resource Kit 