Free online strategy games come in many sizes and flavors. There are free strategy games that you can play in your browser, turn-based games, and real-time strategy (RTS) games. Some games listed below can also be downloaded.

MMORTS
List of Massive Online RealTime strategy Games and retail online strategy games.

MOBA
The best games in fastest growing area of strategy gaming.

Free Online Strategy Games

Pardus 
Graphics: Yes
Download: No, Browser
Real-Time or Turn: Turn-Based.
Free browser game where pirates and space pilots battle it out to be the wealthiest in the universe.

OGame Graphics: No, Text Download: No, Browser Real-Time or Turn: Real-Time Free online space strategy game. Form alliances with other players, perform espionage, and research technologies. Ogre Wars  Graphics: Pictures only. Download: No, Browser Real-Time or Turn: Turn-Based. Free forum game that is played directly on GameOgre.com

Allegiance
Graphics: Yes
Download: Yes
Real-Time or Turn: Real-Time
3D space shooter/sim that requires requires teamwork from other players to advance in the game. 

Star Supremacy
Graphics: Yes
Download:No, Browser
Real-Time or Turn: Turn
Science fiction colony management and galactic conquest game reminiscent of Masters of Orion. 

NavyField  
Graphics: Yes
Download: Yes
Real-Time or Turn: Real-Time
Massively multiplayer online tactics game or MMOTG for short that you can play for free.

TinyWarz
Graphics: Yes
Download: No, Browser
Real-Time or Turn: Turn-Based
Players build vehicles, create armies, deploy bases, then battle other players and computer controlled bandits on distant worlds. Each player is represented by a commander which leads your army.  

Bang! Howdy
Graphics: Yes
Download: Yes
Real-Time or Turn: Turn-Based
The makers of Puzzle Pirates try to bring the same charm to the old west that Puzzle Pirates brings to the high seas. 

Space Civilizations
Graphics: Yes
Download: Yes
Real-Time or Turn: Turn-Based
You share a galaxy with hundreds of other players in this free online turn-based strategy game. The game has a very slim downloadable client.

Hyperiums
Graphics: Little
Download: No, Browser
Real-Time or Turn: Turn-Based
Trade, spy, fight, and use diplomacy with other players in order to have the best space empire.

WarOnline.net
Graphics: Yes
Download: No, Bowser
Real-Time or Turn: Turn-Based
Free Online RTS/turn-based strategy game that uses an interactive graphical map.  This game is browser-based and allows both campaigns and instant battles. 

Darkwind
Graphics: Yes
Download: Yes
Real-Time or Turn: Turn-Based
Massive strategy game that boasts racing and vehicle combat. 

Starport: Galactic Empires
Graphics: Yes
Download: Yes
Real-Time or Turn: Real-Time
Free starship game where you compete against other captains for two weeks and then see how you did.

Bushtarion
Graphics: No
Download: No, Bowser
Real-Time or Turn: Turn-Based
Free online strategy game where you head up your own gardening company and ally or war with other companies.

Earth 2025
Graphics: No
Download: No
Real-Time or Turn: Turn-Based
Free strategy game that boasts over 50,000 players. You try to be a great conqueror by using economics, politics, and military force.

Space
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
Free web-based space strategy game with a standard and deathmatch server. 

Shadowmere
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
Online strategy game with 17 races and several different ways to attack other players in your quest to be the game's most powerful player.

Neveron
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
Free online empire strategy game where you try to outthink and outmaneuver other empire builders.

Race War Kingdoms
Graphics: Yes
Download: Yes
Real-Time or Turn: Real-Time
Combines PvP combat, strategic warfare, and economics into one solid free online game. 

BattleMaster
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
Strategy RPG that is best played when you have spare time.

Mech Crusaders
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
Build your own army of mechs and unleash havoc. 

Dark Galaxy
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
A space empire building game where you manage your planet or planets while trying to capture other players' planets.

Ferion
Graphics: Yes
Download: No, Browser
Real-Time or Turn: Turn-Based
Free space sim where you are in control of such features as trading, making alliances, and building your own fleet. No download or installation because you play it right in your browser.

Kings of Chaos
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
Very popular online strategy game where you can actually build your army using Internet links. 

Galactic Quest
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
A free web-based online strategy game where planets vie to be the very best in the entire galaxy.

Subspace Continuum
Graphics: Yes
Download: Yes
Real-Time or Turn: Real-Time
Long-running multiplayer space combat game.

Ages of War
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
Fantasy war game where you use magic, espionage, and simple military strength to conquer your foes. 

Hostile Space
Graphics: Yes
Download: Yes
Real-Time or Turn: Real-Time
Play as a starship captain in a persistent galaxy. Jobs as a captain include smuggling, bounty hunting, mining, and space combat. 

The Reincarnation
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
Free online strategy game where you take the role of a mage and command your own army.

Alliance Wars
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
Free space strategy game that you play in your browser.

Hostile Online
Graphics: No
Download: No, Browser
Real-Time or Turn: Turn-Based
PvP browser game where you build up your assassin and learn skills such as blacksmithing, chemistry, and excavation.