Has a bunch of cool changes, adds a DM at the start to balance things, adds a car follower that you can ride in. Very messy in its execution, though, and the dialogue is sometimes poorly written. Good for a quick play-through.
A lot more serious than Car-Arcanum, adds a bunch of new towns, makes Cattan visitable, and a whole lot of quests to play. Still in progress, though. If your looking for a solid, serious, expansion, then I would recommend this.
Both of these are changes made the to main Arcanum module. If your looking for a stand-alone mod, I would say Doomsday, Vormantown (included with the game), or Lost Dungeon Of Souls.
Working on a mod for arcanum