Netrek

"Netrek is a multiplayer battle simulation game with a Star Trek theme. Up to 16 players are divided into two teams representing the Empires that fight for dominion over the galaxy."

Free to play indefinitely.

Netropolis

"Netropolis is an online multi-player strategy game where you match your wits as a corporate gangster/tycoon against other real people. Buy some land, set up a few businesses and watch the money roll in. Easy, right? Not in Netropolis."

Free to play indefinitely.