Hmmm. At the moment that I checked,
http://www.bd-fans.com/RnD.html was offline, so I found it on archive.org.
Then I tried to download from archive.org all 222 files linked from the main page; and successfully acquired 188 of them.
Only then did one of my attempts to reach the original site succeed. So then I tried to download the 34 files which were not found on archive.org; but unfortunately, the apparent reason that archive.org doesn't have them is that bd-fans.com also doesn't have them! I did a spot-check on a file or two which
were on archive.org, and they were also present on bd-fans.com.
The 34 that I failed to download were as follows. (Note: some of these are of only tiny historical interest, like builds for OS/2 and PocketPC; and some others are readily acquired elsewhere, including artsoft.org; yet finally, there also appear to be a couple dozen which might be unique and therefore currently 'lost'. I marked with '(*)' some items below which I'm pretty sure are in 'standard' locations like artsoft.org or even, indeed, in the basic RnD downloadable packages). See the archive.org and/or bd-fans.com pages for readable descriptions:
Files/RnD/Graphics/MBPGS.ZIP
Files/RnD/Levels/42_Steps.zip
Files/RnD/Levels/ASiekierkas_Showcase.zip
Files/RnD/Levels/BD2K3.ZIP (*)
Files/RnD/Levels/Boulderdash.zip (*)
Files/RnD/Levels/Classic_Supaplex.zip (*)
Files/RnD/Levels/Colourful_World.zip
Files/RnD/Levels/Contributions_2005.zip (*)
Files/RnD/Levels/Element_follows_element.zip
Files/RnD/Levels/Emerald_Factory_%28Easy%29.zip
Files/RnD/Levels/Fool_that_bug.zip
Files/RnD/Levels/JuergenBonhagen.zip (*)
Files/RnD/Levels/Level_in_level.zip
Files/RnD/Levels/MP-Richard.zip
Files/RnD/Levels/Manuels_Sokoban_Levels.zip
Files/RnD/Levels/Master-Rocks.zip
Files/RnD/Levels/Maze_Solver.zip
Files/RnD/Levels/Moving_Elements.zip
Files/RnD/Levels/Negundo_World_2.zip
Files/RnD/Levels/Quicksand_for_emeralds.zip
Files/RnD/Levels/Random_Level.zip
Files/RnD/Levels/Rotation_Project.zip
Files/RnD/Levels/Scrolling_Text.zip
Files/RnD/Levels/Slippery_Ground.zip
Files/RnD/Levels/Space_Invaders.zip
Files/RnD/Levels/Step_Puzzles.zip
Files/RnD/Levels/Text_Input.zip
Files/RnD/Levels/Tutorial_Aaron_Davidson.zip (*)
Files/RnD/Levels/Tutorial_Niko_Boehm.zip (*)
Files/RnD/Levels/Wrap-Around_Scrolling.zip
Files/RnD/RnD308_OS2_XFree2.zip
Files/RnD/RoNDi_%28PocketPC%29_v3.1.0.zip
Files/RnD/Utilities/Level_Sketch_v2.zip
Files/RnD/Utilities/RnD_Database_v0.9.zip