Good TBS on steam?

Xcom: Enemy Unknown?

Total War and
Civilization series

Civilization

Command and Conquer: Tiberium series.

Might and Magic Heroes VI

Dominions 3 and 4 as of now :D

CIVILIZATION V
GO PLAY IT (you can get a steam key for the game very cheap, since steam keys were once given out for free for those who voted in the CVG golden joystick awards around a month ago)

That game's freaking awesome, but be ready to buy the DLCs (Brave new world and gods & king) if you like the game, coz these DLCs are the ones which make the game much better

For strategy as in "empire building", i'd suggest Civilization 4 with both expansions. There are some awesome mods out there as well :)

For strategy as in "squad-based", i'd recommend UFO: Extraterrestrials Gold

Shadowrun Returns. Though RPG in origin, combat is turn based like XCOM uses.

Fallen Enchantress Legendary Heroes +mods (game with mods for 1000+ hours :D ), Xcom, Kings Bounty games, Civ V, Disciples Reincarnation (On GG to buy), Fantasy Wars, Elven Legacy, Battle Worlds: Kronos and ssoon on steam Panzer Corps Wermacht.

Civilization V is the top-level TBS on Steam.

Frozen Synapse

/thread

Battle Worlds: Kronos is supposed to be good, but I've not played it myself yet. That reminds me, I should try the demo one of these days.

Eador I've heard mixed reports about, and doesn't have a demo.

Death Rally?

Endless Space

Third Berson Shooter

Eador:Genesis
Disciples I-II
HoMM V-VI

Sid Meier's Civilization V,

Endless Space.

Others have nailed it. I'd highly recommend Civ V and Endless Space, as above. Those, I've played. XCOM:EU is on my shortlist.

For slightly older, but still addictive/viable, Galactic Civlizations II is pretty damn good, too, if you like the space theme. As is Civ IV (robust mods).

King's Bounty series. Well... it is an RPG, but battles are TBS, just like in HoMM series.

any game from zeboyd games or spiderweb software for turn based strategy rpgs and expeditions conquistador

Civilization 4 > Civilization 5

Civ 5,XCOM,Blood bowl,Fallen Enchantress

Civ 5 > Civ 4 >:(