I have written some example code that illustrates one simple method of selecting and highlighting 3D objects in OpenGL. It uses gluUnProject + ray-sphere intersection for the selecting and stencil testing + wireframing for the highlighting. Not very original, but in case you wanted to see in action:

selection.tar.gz (336 KB, there's some textures in it)

Oh, and I've only tested it with FreePascal on Linux (use fpc -Mobjfpc -dSOUND selection.pp to compile it). I suspect that some minor changes might be needed to get it to work on other platforms or with the Delphi compiler. And you need JEDI-SDL, too.