Microsoft has been beta testing Kahuna, the next version of Hotmail, for a couple of months now, and the Seattle Post-Intelligencer newspaper has had a look at Windows Live Mail in the context of Gmail and the new Yahoo Mail, which is also in beta. All three use Ajaxing to provide a user experience that is much more like a desktop application than traditional webmail -- though by desktop standards, they are all slow and somewhat primitive.
Windows Live Mail looks quite a lot like the new Outlook, whereas Yahoo's new Mail looks more like the old Outlook Express. This is not to express a preference, of course. I've used Yahoo's new mail and it's good, though I wouldn't go as far as the Wall Street Journal's Walter Mossberg and say it's "far superior" to Gmail.
Seattle P-I has screen shots of both Yahoo Mail and Windows Live Mail. However, the best source of info on Windows Live Mail is Imran Qureshi's blog: he's the Windows Live Program Manager.
Both services are likely to have an impact on far more people than Gmail. In the US, Yahoo has 10 times as many monthly visitors as Gmail -- 66.6 million versus 6.2 million, on comScore figures. Globally, both Yahoo Mail and Hotmail have something like 200 million users each, whereas Gmail is probably around 10 million or so.
