I get to talk to web publishers every day and there are amazing new sites that keep poping up. In the horse world most of us are sadly behind the curve, failing to utilize technology that is sweeping the internet and known as Web 2.0.
That said, there are many sites that are still interesting and purposeful. Here are a few I have discovered (or have known about for a long time):
Ultimate Horse Source: This site has a lot of information, many different people have contributed and the forum is very active. I learned a lot about genetics and color reading these pages. The pages are slow and poorly formatted in Firefox, I have not viewed them in IE.
Dreamhorse: This is THE destination for someone wanting to sell a horse utilizing the web. Honestly I think they are way to restrictive on pictures, it is confusing to have multiple logins (one if you buy a picture ad and one for dreamhorse) and not really taking advatage of new web features. But, it has huge market share and name recognition, so everyone uses it.
Horse.com: This is also the Country Supply catalog, so you can find tons of great stuff there and the prices are great. I would definately encourage you to visit your local tack shop/feed stores and shop locally (good for the economy), but this is a great place to research. I have actually found better prices at my local stores so I haven't needed to shop here in a while, but occasionally they have a killer deal I cannot pass up. They are experimenting with horse sale ads, I'll keep an eye on that.
Thehorse.com: Good information here on horse health. Looks like you just have to be a member now, but they used to charge for full length articles (if I remember correctly). I guess they figured out they can make money from advertising. I wonder if I should contact them about my product RMX Direct. I would hate to mix work and pleasure.