You're aware of the
CAN-SPAM Act of 2003 right?
You can't just go around e-mailing 1000's of people unless they've opted to receive e-mail from you.
That said, you could put up some form of squeeze page where users can opt-in to receive e-mails from you. This legitimate collection of e-mail addresses is called "building a list".
This would be perfectly legitimate. You should have a double opt-in though which means that the user has to confirm their subscription by clicking on a link in an e-mail you send to them.
You should also put an unsubscribe link and your address in all e-mails you send too.
A lot of people use services like
aWeber to manage their lists and send out bulk e-mails.