Free-to-play may be a modern oxymoron in terms of the games industry, but it does mean that AirMech is worth checking out to see if you fancy playing further.  The game has traditional RTS elements as you build bases and units, but it has the added element of allowing you to control a MegaMech that transforms from Mech into an aircraft.

The beta is set to include all components of the main game including single-player, multiplayer, and co-op for up to three players online. 

AirMech Arena is a free to play, action-based RTS where you pilot transformable mechs while commanding other units. The mechs morph from various aircraft into their more combat-focused bipedal forms to blast the enemy's base into oblivion.