UFO 2000
Continuing X-Com week, UFO 2000 is a multiplayer engine that can use the original graphics files for X-Com, or its entirely new set. The mission setup screen has a preview of the generated map, which gives a good overview of how X-Com style random map generation looks in practice, with the maps patched together from large, prefabbed blocks of times.
It has its own map sets and tiles, some of which work in the original game and some of which are entirely new. (There’s some overlap with OpenXcom, as you might expect, since the projects cross-pollinated.)
And, of course, it is open-source, so if you want to take a peek under the hood, you can.