Do you play 1 and 2, or 3?
Then I'm afraid there aren't really any. Descent 1 wasn't really set up in such a way for modding like that to be done--the tools for it didn't exist--so no one really ever did.The most "modded" D1 level (set) I ever saw had a macro-whatever included as an optional install that, when you installed it, it gave all of your weapons new names...but all of the weapons still looked and behaved exactly as they did before. Never have I heard of a custom bot in D1 either. That's about the full extent of modding got with D1.D2 and D3 are a different story though--far more modding took place in both of those games.