Have you tried article marketing? It is a great way to get targeted traffic to build up your list. You don't want just any traffic, but you want ones that will potentially buy.
you can get software that extracts email address but you gota do it legally only extract content which is yours in the first place. best thing to do is.. give freebies away in exchange for email addys good luck
There are some sites where you can get email adresses from but then a lot of these people won't read your mail. You better set up a sign up box on your site and ask people to give their email adress, this way you know for sure that people are interested in your mails.
yeah like i said before, offering maybe a free ebook and regular tips via an email list for the niche, offer for example some free wordpress themes if its a web related site
Offer something for free and drive traffic using PPC. Of course you have to do tests and determine your value per customer so you can determine what you can afford to pay per click or per lead. But once you build a decent size list you can do ad swaps. and easily add hundreds or thousands of new subscribers per day. For example my list is now up over 13,000 subscribers and if I send out a promo for someone and they send something out for me I might get 400 - 500 new subscribers in one day from someone with a 15,000 email list.