This plugin enables AC3D to read and write OBJ files.
Andy Colebourne has taken over development of the ac3d plugin - you can find the most recent version here.
The exporter version 3.2.1 works with AC3d 6.1 or newer, and supports visual editing of animation. Download for:
Really Old Versions
This older version of the plugin works with AC3D 5.0 and 6.0. One download has the Mac and Windows version: