OpenXcom
OpenXcom is an open-source implementation of the original X-Com, capable of using the original data files from the game in a new-and-improved engine. It makes it much more playable, and most interestingly from our perspective, has fairly extensive mod support. Including new map types for the map generator. Some combine the original tiles sets in new ways, while others add entirely new tiles.
It requires a copy of the original game if you want to use the official graphics.