Actually, one place I know uses them as coasters, and another has a tabletop made out of them covered in resin, so I guess this is the third useful thing
After years of uncontrolled MtG addiction, I have so many basic lands that I used them for that, and to scoop up weed to put in the bowl, and to pick my teeth, and to cut into doobie filters, etc etc.
I think an awesome way to use them would be to have the BBEG be a planeswalker. When the fight starts, the DM pulls out a Magic deck and starts playing Magic against D&D.