Glorantha, the original Runequest setting, is flat. Most worlds I run are flat. Flat Earth theories are pretty rare in our own historical past. But if you want to check out real world flat earth ideas, I recommend looking at Chinese mythology and a fabulous book written in the 6th century called Christian Geography by Cosmas Indicopleustes.
Most flat earths don't have edges, in my experience; most have encircling oceans or encircling mountains. A world with sharp edges would be really cool, though -- I look forward to seeing what you come up with.