What are some free rendering plug-ins available for microstation v8i?
None. There is not such idea in Microstation as plugins for rendering.
Build in Luxology render from Modo is nice and fast. Or LumenRT.
You can export to fbx, dae, skp and use for example D5 Render free edition or any other software like Lumion.
Previously we have some connection to Maxwell Render but the software is not free.