If you were to buy it now, you'd not be purchasing the nearly exclusively rights to "e-mail" as it was back then. People would actually say: "Send me a Hotmail." What made it valuable to Microsoft was they they got to capitalize off of the leverage over the largest user-base of any online product of that time. With it they kickstarted MSN.

Today, Hotmail probably pulls in quite a bit of revenue from advertisements, but most of it's value was "spent" by herding the massive group of users into other products, which may or may not have made MS more money.

I think Twitter is today's version of Hotmail, in terms of the concept... except Twitter seems to have gotten much luckier.

What this ingrains into my head is: give access to known technology in easy ways. Hotmail, Blogger, Wordpress, Twitter, Facebook -- they allow people to publish/share information where previously they'd need expertise in IT to do it.