Today we begin rolling out a feature that makes it easy to use Hotmail with any existing email address. Just as you can use Microsoft Outlook with an address from any service, you can now do the same with Hotmail on the web.
Implementing this was fairly straightforward given the architecture of Hotmail, our POP aggregation support, and the "Send As" feature that lets you send mail from any validated email address. We just needed to tweak the way we store and look up email addresses, build the first-run experience, and we had it.
To try it out, you just need to register and validate your email address.