Marvel Heroes is a free-to-play MMO that let's you play as many of your favorite heroes from the Marvel Universe (and some bad guys too). Currently, the game allows you to play all of the 50+ heroes all the way to level 10 to get a good idea of how each will play before choosing to purchase. There are a number of ways to earn free characters by playing in game, and you end up with a few characters on your roster when signing up.
I have every single character in the game and have leveled them all to the level cap of 60 (and prestiged many of them). With the last character added being Ultron (and Angela coming next), here are my top 10 favorite characters in Marvel Heroes.
Hopefully you will check out www.MarvelHeroes.com if you enjoy what you see.