Ponytown is a recent viral affair, that has been buzzing around tumblr and the imageboards in recent months.
Ponytown is a social HTML5 experience. I hesitate to say game, as there are not anything outside of walking around the green fields and socializing with other players. There are an extensive amount of customization around the pony you walk around as, from gender and mane styles, to accessories and all the different colours you could wish for.
It's hard to recommend it for its sheer gameplay value, but in the case of socialization, it is an entertaining little thing. You can switch the tiled playing field between grass and dirt by clicking, but I wouldn't say that counts as gameplay. This is usually where the viral or memetic twist snuck its way in, with people having found their own forms of entertainment, drawing whatevers in the grass for people to behold.
As it is, it's still very much in an alpha stage, constantly getting updated and getting new stuff and features added as we speak. Trees and rocks here and there, a new login system, all small additions towards the right direction.
As mentioned, it is an HTML5 endeavour, so it should work in every major browser. Take it for a spin if you are curious, have some friends to hang out with. I've even seen people use it as a virtual meet-up spot, so that's a very possible option as well. There are both a safe and an unregulated server, so I'd assume it is suitable for all ages.