I was shocked to read this on CNN:

Yahoo dominates the niche, with 52.6 million unique users per month in the United States, according to a February survey by online research firm comScore Media Metrix. Hotmail is next, with 45.4 million users. AOL has 40.2 million paying users.

I would’ve guessed Hotmail just based on its long history.

This is from CNN’s article on Google’s upcoming free email service. The draw for the Google service? 1 Gigabyte of storage and a fancy search option.