Most of the time - Hawken :

Free2Play OpenBeta (pretty advanced)
"Hawken is a mech-based first-person shooter. The game takes place in a dystopian human-colonized planet industrialized to the point of collapse, in which the hunt for resources has become a battle for survival."

Guess this will keep me busy for the next months...

HAWKEN - Official Gameplay (Work in Progress) from 2011
^ Sure looks ands sounds somewhat different now, but essentially the same.