Check out this article by GameHowto.com listing Marvel Heroes as one of the give free-to-play games everyone should play!
"...There’s just so much to do that it’ll take hours and hours before something feels truly repetitive. Beating different super villains, saving that hopeless bystander and taking on a T-Rex are just some of the fun filled activities a hero can enjoy. Give it a try, you won’t regret it, I promise you that."
Full article can be found here: Five Free-To-Play Games Everyone Needs to Try