Howrse is a virtual horse site that is filled with little girls, though there are, for some reason, older people there, and on this site, I consider myself to be old.
It is based on the French site, Equideo, which is exactly the same, only older, better and in French. I would consider it to be one of the most advanced virtual horse games out there, since you can train, breed and compete with your horse, and there's mathematics and planning behind it all that makes it work well. And the people who play it know it works, since there are 497698 players active to date, though it only began in April 2007.
It works on the system that you can play it for free, but if you want fancy things that make your horse look good, or that make it better, you have to pay. You horse can be ranked, based on its skills (which deplete slowly after the age of 25), training, BLUP (a strange and often confusing acronym that is only really important when breeding) and genetic potential. You are ranked depending on your horses ranking, though there is some suspicion that this is not all you are ranked on. If you do something bad, such as posting something rude or against the forum rules, or trying to buy a horse you cannot afford, you lose karma, which is another point scheme. Losing karma is a rather pointless exercise, since it doesn't affect your actual playing ability.
Occasionally there are competitions that keep players on Howrse, and many players claim it to be addictive. There is a forum in which you may ask all questions, though woe betide those who do not follow in the clearly marked guidelines at the top of the forum. Mostly the forum is filled with people questioning the stupidness of their own actions, blaming it on everyone else, such as the amazingly common post of "I don't have enough money to pay the vet, will I lose my foal?" This is a stupid question because the answer is put everywhere by practically everyone! You will lose your foal, and one karma point, thank you ma'am, please come again.
Passes are what you have to pay for, and with them you can get coats to make your horse look good, wings to the same purpose, special saddles, bridles, shoes, you can change the gender of your horse and much, much more!
Some people get into this game way too much, creating their profile page to make it look fantastic, spending ages on their horses. Some players have as many as 350 horses, and since I spend about 10-15 minutes looking after my three, these people have way too much time on their hands. Also, because it is a site visited predominately by girls, the profiles are sparkly, and the auctions are filled with comments such as "I don't want to sell him (amazing, since its a girl), he's so cute (the same generic picture is used for every horse of that breed), he'll make a good stud one day (again, wrong he, won't make a good stud, he's nearly 10 and has no training), I didn't mean to buy him. (How stupid are you?)" I am a girl, but I am in the minority, since everyone else seems entirely incapable of reading the stuff right under their noses.
Other then the general population of the site, who seem to think that in no way are they responsible for helping anyone else (despite what I have said above, I always manage to plaster on a smile and help anyone who asks), Horwse is a good site, if you can really be bothered to click lots of buttons to make stars appear over several days. But it is better than most virtual horse sites out there that I have seen.
Update:
These days there is also a German Howrse, which is obviously populated by English speaking people; one week into the existence of the game a message appeared on the forum begging people to speak only in German. See
www.howrse.de if you really want to play.