For me the most popular email provider is Yahoo!. It's rarely observed that people using other free email service provider like Hotmail, Gmail (though becoming popular too),Mail, AIM, GMX and etc. When you asked your newly found friend for his email account about 90% would say email@example.com or y.com (short for yahoo.com). Maybe, it's just here in the Philippines because per region/country it really varies.
When I first use the Internet way back in college (yah! i was already in college when i first surf the net that was 1998) and my first email address was from Mailcity (now owned by Lycos and not free anymore). By the way, my hotmail account was firstname.lastname@example.org (not really sure, but i used Rob Thomas of the Matchbox 20 as my netname).
But do you know that Hotmail was the first free email service provider? Yes! It's true. According to my research, way back 1996 it was the first to offer a free email account with @hotmail.com address and of course webbased. Before, sending email is quite cumbersome because a lot of stuff will be needed to setup and it cost a lot. Your ISP would give you a client program where you can access your email and will be configured by them. But after a year it was bought by Microsoft for more than $400M.
Unlike today, setting up an email is quite easy that even an grade school student could do. You just have to type from your browser the address of your desired email provider like Hotmail, Yahoo, Gmail and etc. and sign-up for an account.
Somehow I would still consider Hotmail hot because at least it would reach below top 10 websites from popular web stats sites. In a review here, Hotmail is next to Yahoo! like in search engine market =). Anyway, it doesn't matter what's your email service as long as we'll be able to exchange email.