Marvel Heroes is a free to play online RPG game released in 2013 by the developer Gazillion Entertainment and Secret Identity Studios. They have the rights to use the Marvel license in an online game, and they made one.
The game plays similarly to Diablo and other isometric action-RPGs, and takes most gameplay aspects from that series. The combat is action-packed, the leveling is traditional like Diablo, and there are gazillions of loot to pick up and use. The story is driven forward with missions and hand-drawn cutscenes. There are many Marvel heroes to select a character from, including Superman, Hulk, America Captain and much more. Some of these are available right at the start, and others have to be unlocked via real money transactions or certain in-game achievements.
The game can be played in solo or multiplayer with Co-Op and PvP modes available. Lore-wise it doesn't acknowledge the fact that there are tons of Supermans, America Captain and more running around in the city hubs. This can also be confusing in PvP or Co-Op when there is more than one of the same character on the field.
The item shop in this game is purely optional, with focus on additional characters and minor content, none of which is a requirement to enjoy the main game.