Google’s Gmail keeps the interface clean and simple. There is a small one line text based add at the top of the email box frame. The user interface is simple but not usable as the Windows Live Mail in my opinion.
Microsoft Windows Live Mail has two huge banner ads one at the top and one down the right-side of the window. These are very intrusive, distracting and take a lot of screen real-estate which requires you to do a lot of scrolling. I do prefer the functionality of Windows Live Mail but the ads are a big turn off. If they could make the graphic ads a bit smaller, I’m sure I would be more willing to use Hotmail.
Yahoo Mail Beta has a larger banner add on the right-hand side of the screen and smaller ads in the bottom left corner of the mailbox. While the banner ad on the right is a little intrusive, it’s still not bad enough to make me not want to use Yahoo mail. The nice thing about Yahoo mail is that if the banner ad doesn’t fit comp[lately in the window it doesn’t give you scroll bar for the ad like Windows Live Mail does. The other nice thing about Yahoo mail is there is no banner ad at the top of the Window. I also like the Yahoo Mail beta user interface. Although I don’t like the idea of having to login to yahoo mail to read my RSS feeds from the mail client.