Why are you sitting on your arse reading a website when you could be sitting on a dinosaur while wearing a cowboy hat and shooting lasers? Why do I even have to ask a question like this?
Dino Storm is a free-to-play browser-based massively multiplayer online role-playing game in which thousands of players wander about a Western sci-fi setting on the backs of giant reptiles. They perform quests and earn levels and fame, all in the name of getting to ride even bigger dinosaurs.
There's much more info at the game's official website, but what more do you really need? Dinosaurs. Lasers. Free. Those are the four basic food groups, even if there are only three of them. Splitscreen Games should theoretically have been able to just flip the launch switch and instantly have all the players.
It's rather fun to play as well, if that matters.