If you liked the classic Ogame before all the p2w stuff was added, you'll like this game.
OGame - OGame is an addictive, Massively Multiplayer Online, Real-time Strategy, Space-war, and Money Management Browser-based video game develop by Gameforge AG.
Galaxy on Fire - Galaxy on Fire is a space combat and trading simulation.
EVE Online - Player-created empires, player-driven markets, and endless ways to embark on your personal sci-fi adventure.
SpaceHunt Game - SpaceHunt is a chat-based multiplayer adventure similar to Ogame and StarCraft: build a base and go on missions to explore the universe and attack other planets to gather resources and dominate the Universe.
Entropia Universe - Entropia Universe is an Addictive game for those players who love playing MMOFPS games.
Gratuitous Space Battles - Gratuitous Space Battles is an Adventure-based, Real-time Strategy and Single-player video game developed by Positech Games.