Real-time graphics with Blender
You can produce real-time graphics with Blender in two ways. You can use Blender's internal game-engine or you can export models and use externel rendering libraries
topOpenScenegraph
The OpenSceneGraph is an open source high performance 3D graphics toolkit, used by application developers in fields such as visual simulation, games, virtual reality, scientific visualization and modelling. Written entirely in Standard C++ and OpenGL it runs on all Windows platforms, OSX, GNU/Linux, IRIX, Solaris, HP-Ux, AIX and FreeBSD operating systems.
I have written a small tutorial how to make models with Blender to the OpenScenegraph-engine.
topCrystal Space
Crystal Space is a free (LGPL) and portable 3D Game Development Kit written in
C++.
Crystal Space currently runs on GNU/Linux, general Unix, Windows, Windows NT, and
MacOS/X.
Ogre
I haven't tried Ogre myself yet. So I can just offer some links: