I have managed to convert all eight levels (plus one scrapped level) of an old ball-rolling game, The Last Smiley, into MB levels. (And I've waited 16 years for this to be a reality!)
See all levels: [Pack] The Last Smiley - Converted levels by evaneezer - Marbleland
See also: The Last Smiley sounds, skyboxes and marble skins
3/13/2025 update: All 3 levels from another 3D game, Jimmy Neutron Invention Revenge, have been converted and formatted into Gem Madness levels. You may need to lower the graphical detail (in Options) slightly to prevent PQ from crashing, it happens sometimes. Hope you enjoy the converted levels!
Link: [Pack] Jimmy Neutron: Invention Revenge - converted levels by evaneezer - Marbleland
Special thanks to RandomityGuy for helping me find a handy tool that converts .w3d files to .obj files, thereby allowing me to convert those to MB interiors.
My previous levels:
- Eight Flags
- Campground
- Ice Cream Cart