Here are a couple of screenshots from an OpenGL-based 3D game engine I've been coding in my spare time for a while:
Some if its features right now are fully dynamic lighting, md2 model loading, stenciled shadow volumes (not shown in either of these screenshots because my ancient video card, a Voodoo3 2000, doesn't have a stencil buffer, and it's too slow in software rendering to get a good shot), particles, smoke, heightmaps, and simple AI (you can see two characters fighting eachother in screenshot 2). It's written in C++ and uses SDL for input and such.
Maybe some day I'll get around to releasing something based on this. The only bad thing about 3D/game development for me is that I can't create very good art to use (the model used in screenshot 2 is the "rhino" quake2 model, which I got from polycount, and all of the textures were made with GIMP filters). Oh well.