There are several plugins that integrate Mitsuba with external modeling software. This makes it a breeze to create complex scenes without having to deal with Mitsuba's native XML scene description format.
Keep in mind that the exporters on this page are still rather experimental; currently, only a subset of Mitsuba's features is supported by them.
About: The Blender plugin is developed and maintained by Bartosz Styperek and Francesc Juhé. It currently supports creation of triangle geomety, participating media, as well as editing of surface and subsurface scattering models, along with previews inside Blender. Some of the "wackier" modes of operation of Mitsuba are not implemented.
How to install: The current version of the plugin is available in the mitsuba-blender Mercurial repository (download a ZIP file of the latest version here). Unpack this file and move the resulting mitsuba folder into your Blender scripts/addones folder. After restarting Blender and enabling the plugin in the addons preferences, you should be able to select Mitsuba as the currently active renderer. In the main scene tab, you will need to set the path to the main renderer executable (mitsuba.exe on Windows, Mitsuba.app/Contents/MacOS/mitsuba on MacOS, /usr/bin/mitsuba on Linux).
About: There is currently a very experimental integration plugin for Rhinoceros version 5 on Windows. As of yet, only geometry is exported (i.e. no materials), but at least this part works very well. The plan is to extend this to a full-featured exporter with material selection and preview.
How to install: Everything is described here
Rhinoceros is a trademark of Robert McNeel & Associates