Would recommend that you look at the Free to Play page on steam under the games tab I'll drop a link here: http://store.steampowered.com/genre/Free%20to%20Play/?snr=1_200_200_Free+to+Play_12
The few that I enjoy on there are Team Fortress 2, Load Out, DoTA 2 and Thinking with Time Machine which is a Portal 2 mod. Enjoy