Armored Brigade is freeware, though there is a commercial version in development. But it's already very playable and will keep you going for hours. Or if you like flying, try falcon 4.0. Just be sure to get the 4.33 bms modernization from the modding community.
Command: Modern Air/Naval Operations is one of my all time favorites. The title says it all.
Steel Beasts Pro PE: Tactical Tank Action. And MAPS.
Armored Brigade: Amazing little gem. Cold war era mechanized warfare. And it's free.
Then I guess I should count myself lucky, being able to find one that lasted over a decade and a half, enjoys a stable (if slightly small) population and has dedicated and talented developers.
Personally, I have found that F2P games provide the best experience. Not just any F2P MMO, but a nonprofit one. One that subsists on donations and personal investments. In other words, the RPN model. As making money is not the objective of any involved party, the entire profit dilemma suggested...