Neptune's Pride 2 is described as 'A board game for the web! -Login throughout the day to command epic fleets of spacecraft in a desperate struggle to conquer the galaxy! -Perfect for people without much time, just a few minutes a day is all you need to command your empire'. There are eight games similar to Neptune's Pride 2 for a variety of platforms, including Windows, Linux, Online / Web-based, Mac and iPad. The best alternative is Civilization. It's not free, so if you're looking for a free alternative, you could try The Battle for Wesnoth or FreeCiv. Other Neptune's Pride 2 like games are TripleA (Free, Open Source), Warlight (Freemium), WebDiplomacy (Free, Open Source) and OpenFracas (Free, Open Source).
Freeciv is an open-source, turn-based strategy game based on Civilization II. Inspired by the history of human civilization, the game commences in prehistory and your mission is to lead your tribe from the stone age to the space age...
TripleA is a free (under GPL) turn based strategy game based on Axis & Allies board game. A wide variety of map scenarios have been developed for TripleA (World War II, Punic Wars, Napoleonic wars, etc.
webDiplomacy is a multiplayer, online version of Diplomacy, the popular turn based strategy game in which you battle to control Europe; to win you must be diplomatic and strategic. You control armies and navies and attempt to overthrow the other world powers.
OpenFracas is a free, open-source game that is similar in style, but not in specifics, to Risk. On randomly generated and customizable maps, humans are pitted against other humans and AIs in an attempt to eliminate all other players or teams.