I am working on an OpenSim/SecondLife compatable viewer. I almost have it to a state that I am going to let the BSD Licensed code loose. I want to get it to the point of being buildable and runnable and not totally sucky before I put the code out. That's not to say it will be anywhere usable or feature complete at that point.
The biggest problem at the moment is performance. I have clearly made some of the noob mistakes when creating a visual application. It disparately needs a manager and scheduler for all of the work queues. The current implementation gets totally overwhelmed when entering a large sim (especially an OpenSim sim since the whole sim contents is thrown at the viewer).
Here are some progress pictures. These are of the Portland Connection sim in SecondLife(r).