is the homepage of a very fine game. Units don't stack (in the vanilla version*), but the game is hex-based and utilizes heroes and units. Even normal units can level up, be renamed, and be used from one battle to the next. The vanilla version has several very well developed quest-lines.
Battle for Wesnoth is completely open-source and very easy to mod. (I once used mechs for units)
Development is ongoing... it's not a dead community or game. Also, it has been around for years.
Due to the fact that it is free, open source, well developed, old code, and well maintained... it is highly moddable. There are tons and tons of mods and quest packs out there, so you can play it again and again... and if you start to get bored, try out a new mod, quest, or multiplayer game-type. There are dozens of active servers up 24-7 hosting a number of games at any given time.
*vanilla means un-modded (for anyone who doesn't already know)