What are some alternatives?
When comparing Galaxy on Fire and Minetest, you can also consider the following products
EVE Online - Player-created empires, player-driven markets, and endless ways to embark on your personal sci-fi adventure.
Terasology - Terasology (started under the name Blockmania) is an open source project started by Benjamin...
OGame - OGame is an addictive, Massively Multiplayer Online, Real-time Strategy, Space-war, and Money Management Browser-based video game develop by Gameforge AG.
Voxelands - The Fun-Focused Free Software Voxel World Game.
Vendetta Online - The space-based MMO Vendetta Online balances an online trading system with tense ship-to=ship combat.
Terraria - Dig, Fight, Build! The very world is at your fingertips as you fight for survival, fortune, and glory.