Update your email signature to include an attractive catch phrase and your link.
"PS: I love you. Get your free email at e-mail at Hotmail" - Hotmail
This signature helped Hotmail get about 3000 users/day, and they got to 1 Million users by 6 months. 5 weeks after that they got to 2 million users.
In Bhatia, a friend sent an email to another friend using Hotmail, then in 3 weeks, Hotmail had 300,000 users there. By 1.5 years, when they sold to Microsoft, they had 12M users.
(Thanks to Mattan Griffel for this information)