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CIV IV
sorry had to say it :D

Aerena - Clash of Champions

Total war: shogun
Haven't played it, but I saw it got good reviews, but it's not F2P.

There's XCOM: Enemy Unknown but it's not F2P.

The Battle of Wesnoth. And it's even free! :D

Heroes of might and magic 3 used to be one of my favourite games.

Risk?

Sid Meier's Alpha Centauri/Alien Crossfire.

It's old, not on Steam, you can get Alpha Centauri on GOG. I have the original CD's for both, I still play it to this day. You can even play by email.

I wish they would develop an Alpha Centauri 2.

King’s Bounty: Legions
Turn based and free to play.

skulls of shogun, not f2p, but if you get 75% discount its good price

Affected Zone Tactics - squad based f2p tbs & Lost Sector Online - tbs f2p. Panzer Corps - from torrent, play lan via tunngle.

My favorite was warlords III darklords rising, wasnt free at the time but I guess there are ways to get it "free" now.