law56ker has released a huge collection of Doom mods and total conversions, running on nxDoom, on 2 CDs.

This is a collection of best Total Coversions, Megawads, and Various levels 
for use on NXDOOM for the Sega Dreamcast. I spent the last several weeks, 
researching and collecting the best quality mods, megawads, levels for Doom 2 
and Ultimate Doom. It was a lot of work collecting and testing all of these 
mods, but the true work went to the people who made them and those who made it 
possible to play Doom on dreamcast.

All you need to do is put your retail Doom WADs in the files and use his instructions with BootDreams to build the disc.

Source: DCEmulation Forums