RealXtend is an open source project which has extended the feature set of OpenSimulator to support normal 3d meshes etc. which have not been available in Second Life(tm). The core development is funded and currently organized by Oulu Innovation in Finland, as a cooperation by three Oulu based companies Evocativi, LudoCraft and Playsign. The development process and decision making is open on mailing lists and irc like normally in open source projects, and any OpenSimulator or other developer is welcome to participate, and can get commit rights etc.
Information on how to connect the RealXtend viewer is defined by the ModRex interface. That page also gives more detailed information about the features.
Originally (in early 2008) RealXtend published an improved version of the GPL'd Second Life(tm) viewer which can be downloaded here. The viewer for Windows is prebuilt. Here are instructions for running RealXtend in Wine if using Linux.
Now in 2009-2010 the focus has been on the development of a new viewer, Naali, which is compatible with the existing servers and content, but is written from scratch to facilitate development of new features. Naali is Apache licensed (BSD-like) and applies the Opensim contributions policy, so Opensim server side developers can participate in the viewer development too. http://wiki.realxtend.org/index.php/Getting_Started_with_Naali