I'm trying to find mods for this as well, and yes, both of the featured links are down / out of circulation. :\
If possible, perhaps at least update the link rawmilk905 gave into the OP? It's the correct adjusted link for Descent 3 Sector (apparently some server updates broke the old system -- that's what their news feed mentioned -- hence the changes).
As well, I came across Sol Contingency, which looks to be an attempt at revitalizing Descent-like gameplay. Unfortunately, it's team seems to have gone dark, and isn't actually a mod, so no luck there.
The only thing on moddb is a link for BaseWars... with no files or information or anything.
Nexus Mods has zilch, though that was somewhat expected.
Many other places are either 502's or dead in the water :(
It's sad to see such a game getting so little attention in recent years. Then again, the modding scene is really crammed, and the Descent games are rather old and were fairly niche to begin with.
At least Descent Underground seems to have passed Kickstarter, but appears to be multiplayer-only, which clashes with my desire for single-player. :(