http://www.realmofthemadgod.com/

Been recently playing this game. It's sort of a miniature MMORPG, with leveling up to 20. See for yourself - it's quite simple.


Some spoilers and tips for new players:
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If you want to level fast, you can use the command /goto *player name* to teleport to that player. Choose large groups of players from the map (you can see the names in the map) and follow them around. It's a bit dangerous, since the monsters can sometimes oneshot you, and death's permanent, but if you're careful, and with a good group, you can easily level to 20 in less than an hour.