Milk Carton is a Source-engine based multiplayer mod featuring extensive weapon customization. So extensive that is does not even customize existing weapons but allows the player to create their own guns.
Full Version of Milk Carton v1.1. To install, extract into your steamapps/sourcemods folder, (overwrite any existing files).
Nice, what's new?
Look at the news page. The feature list dropped a little earlier this week.