If you're looking for a superhero MMO but don't want to pay for DC Universe Online, then you should know Champions Online is free-to-play.
If you've been curious about Champions Online since it was released back in 2009, but never got around to playing it, then now is the time to check it out. As of today, the game is officially free-to-play, with optional subscriptions (and discounted lifetime subscriptions, too).
As you can see in the new launch trailer, Millenium City is under siege from just about every threat imaginable (including demons and giant robots). Since the other Champions are fighting threats across the globe, Defender puts out a call for aid to anyone who can hear him.
If you're tempted to help Defender out with his battle (or if you have an old account that you're tempted to reactivate), then head over to the game's official site and start fighting the good fight.