Thea Render has released a Blender add-on which provides tight integration into the Blender workflow.
From the announcement:
Thea Render is an exceptional quality external rendering solution that is now integrated on Blender as tight as possible. Features such as interactive rendering, materials editing, instancing support make it possible to work inside Blender creating astonishing results with a smooth workflow between Blender and Thea Render. In addition, we have taken the decision to release the plugin under GPL (open source) that, we believe, fits the spirit of Blender development itself.