Well, i dont know of any FREE online RTS games. I do know a few that you can prob get for $20 or less by now.

Sudden Strike (Sudden Strike Forever Addon)
Sudden Strike 2

Both a RTS...each unit is represented. MP, capturing balloons is only way to get reinforcemnts. IE, they are objectives on maps. Capturing all 2 (or 3) of the same color balloon gives you specific reinforcemnts (Artillery, Tank, Infantry, Katuysha, etc) that can turn the tide of the war. Its a matter of defending the balloons and capturing them. No base building, all unit base. You only have what you start with until you capture balloons for reinforcements.

Company of Heroes is another one, though you have to build buildings on the map (though you can convert a town building into a forwards-barracks). Pretty good game too.

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