[PC] Free: Total Annihilation: Commander Pack at GOG

To claim the freebie:
Go here and click "Get it FREE" in the Total Annihilation banner on the GOG homepage. (You may have to scroll the page to find it.)

About the game:

What began as a conflict over the transfer of consciousness from flesh to machine has escalated into a war that has decimated a million worlds. The Core and the Arm have all but exhausted the resources of the entire galaxy in their struggle for domination. Choose your side, build an army, and crush your enemies on huge, three-dimensional battlefields. And remember, there is no such thing as "too many guns"!

For reviews, system requirements, etc:
Refer to the game's pages on GOG.