Secret Development is a relatively new mod, that starts in an old abandoned factory, renovated by the Combine. They're using it to develop secret machines for a possible extinction of the human race, like street hovercars, giant cannons, etc. You, led by Robert Johnson, have to stop the production lines before humanity is doomed.
This is the first (and only) screenshot I have (uploaded at The Snarkpit) 
If I get enough positive feedback from this mod (when it's finished, of course), I might start a sequel to this mod.
What would a mod be without custom-made content? I've just recently learned about making and compiling models, and I now have 3DS Max, so I'm happy :).