Microsoft Live.com

Live.com is a work in progress, built on the back of Microsoft’s original demo start.com. With Live.com, users add ‘gadgets’ to their page. Gadgets are basically mini applications - like widgets in Konfabulator (now owned by Yahoo). Currently the gadgets on offer in Live.com are the usual set, which most if not all of the other AJAX homepages also offer: mail, stock quotes, weather forecasts, horoscopes, ticking clocks, quote of the day, etc. But there are signs that much more functional gadgets are on their way - for example a tv recommendations gadget, which talks to your Media Center box in order to program tv shows. 181 gadgets have been built to date on microsoftgadgets.com, with many more to come.
Also Live.com has an open API and looks to have a growing developer base for gadgets. This is going to be the key for survival for any AJAX homepage - offering APIs and enabling developers to build gadgets/widgets for the platform.
As the "homepage for windows live", Live.com will undoubtedly have a huge user base and probably developer base too. This is the one to watch, unless Google (see below) can up the ante.