[Edi]

The RMG has gotten fairly little attention compared to mods and other things with higher priority. I am not very familiar with it myself, so all the shortlist entries regarding it are based on entries by other people.

The RMG is a useful tool in creating new maps, but you practically have to open the newly created map in the map editor and then check terrain and neighbor assignments. Currently it is a good idea to restart Dominions 3 every time after you've created a random map to avoid problems stemming from a corrupted geotable (scales bug, but would explain a lot of RMG strangeness).

You should also probably take a look at the maps and mods subforum, there are a fair number of user created maps there and there's also a sticky thread that links most map threads, so it shouldn't be too difficult to find them. Most maps are medium-sized, but there are some small ones as well.