The game I've played is for Android and iPhone, and it is one of those facebook-style games where you have minigames and you basically build your own ponyville. I haven't really played it in a while though. I know there are some other games but I haven't tried those yet.
And I know, right? It's really addicting. When I first heard about it I was like "Why is everyone into MLP? But then I got bored one night and decided "I'll watch the first episode to see what it's like" and then I was like "Well, that's a 2-parter, so I'll watch Ep 2 and see how it ends" then I was like "Well, to be fair, maybe I should watch a regular episode since that was the Pilot episode"
* 3 hours later *
Me: "Myy little pony! Don't you know you're all my very best frieeeeeeeeeeeeends!" ... "What just happened? Did I.. just become a brony?"