I'm completely addicted to this one (and have been for over a year now lol). It is a own/breed/show type of online horse game and so far is my favorite =) I'm working on a bio/genetics major in college and love the realistic genetics of the breeding in this game and the active in-game forums. I'm including a link here if anyone wants to check it out. I am player #70712 (Hokiegrl09 on in-game forums) if you join and want to find me.
HuntAndJump.com: Free Horse Genetics Game