There are quite a few online mattress stores where you can.

US-Mattress.com

When it comes to mattresses, they do have a variety of trusted brands to choose from such as Seaaly, Simmons, Serta and Bassett. Customers can easily navigate the site by choosing different brands. However, I couldn't find an easy way to buy mattress by other factors such as price range and comfort level. On the product details page, it has a price match guarantee to show they offer the lowest prices. I have also seen this by a lot of other vendors and my understanding is that each vendors carry different lines of mattresses and even for the same one, they give different names. As a result, it is very hard for you to find the exact same one and compare the price online. Consider this to be one of the "tricks" played by online stores. If you don't like the one you are currently browsing, they also recommended the similar ones that are viewed by other customers.

The good: A wide selection of mattresses, detailed information about each listing.
The bad: Better navigation system for customers.

Mattressdirectonline Comparing to other mattress online stores, this one does not have a professional web 2.0 design. They group the mattresses by comfort levels, such as firm top and pillow top. Although this site provides free delivery, it is not free white glove delivery, which is the free service provided by US-mattress.com. When you look at the product detail page, you can't even find a price tag. This is a big no no for online e-commerce. My guess is that you will need to call them to find out the price? As a customer, I certainly will not shop here. Moreover, the information they provided is limited. I tried the online chat feature, but seems it never worked for me and you will have to leave an email if you have any questions

The good: Simply design
The bad: limited product selection, limited product information online, needs better web design

More reviews coming...