Chameleon - Grasshopper & Revit Interoperability

Chameleon is a plugin for both Grasshopper and Revit with a focus on interoperability, simulation, and efficient practice workflows. Chameleon’s main advantage is its ability to facilitate easy transfer of geometric data between Grasshopper and Autodesk Revit, but also includes other valuable tools to make life easier, both in Grasshopper and Revit.

There's more information on this free add-in on Hiroshi Jacobs' website.

Further reading: Grasshopper-related add-ins.