[Update: You also do this with the Firestorm viewer. Just Export your prim object using the Collada format. Details here on the Firestorm wiki.]
This is pretty cool.
The most recent version of the Singularity viewer (version 1.8.1) adds a particularly interesting feature:
- Wavefront (.obj) and Collada (.dae) Export by Apelsin, Inusaito, and Latif Khalifa – Allows export of your creations into Blender, Unity3D and other modeling applications and game engines
This means you can now take a prim-based object from within Second Life or Opensim and export it to your hard drive as a mesh object (either .obj or .dae file format).