Windows Phone 7 is soon to be released. Specifically, October for the EU, and November for the US.
For those that haven't checked out Windows Phone 7 already, it looks great. The apps are slick too (check out the foursquare video for example). It's all based on Window's new design paradigm, called "Metro". The WP7 homescreen presents you with a bunch of dynamic tiles as a starting point, allowing you to see everything that important to you at a glance.
Despite it's shiny new look and feel, whether it will be able to compete with the iPhone's massive following of users and developers has yet to be seen.