MegaGlest is a 3D real-time strategy game (RTS) with medieval and magic character which offers tutorials, scenarios but also customizable single-player and multi-player skirmishes.
Multi-player mode supports cross-platform matches up to 8 players over the Internet or LAN against human opponents, or in cooperation against CPU (A.I.) players of various difficulty levels.
From within the game you can download a lot of additional game data (e.g. other maps) at no cost.