Using software or blackhat methods to get an email list is a waste of time and money in my opinion, the emails you get will be close to useless, unresponsive to you because they don't know who you are, and will likely mark you as spam and make complaints.
Remember behind the email addresses are real people who will spend real money on a real solution for their problems. It's just as important to spend the time building a relationship and building trust with your subscribers as it is building your actual list. A list of 20,000 is useless if you can't get any of them to open your emails or click on your links. On the other hand I have made thousands of dollars from a list of roughly 600 subscribers.
It depends on what niche you're in, but in general some of the best list building methods I use are:
- Opt in forms on my blog sidebar and ad bottom of articles with a call to action.
- Sending all my traffic to my squeeze page first, making building my list my #1 priority
- Solo ads. Buying solo ads or newsletter sponsorship from bloggers and internet marketers in my relevant niches and sending them to my squeeze page is one of the quickest ways I build my lists.
- Online marketplaces. You can find these in forums and websites relevant to your niche where you can list your product or service for free or for a fee. I post a listing about my free offer with a link to my squeeze page where they can opt in to download the free offer. This is another great way to build a list.
Those are just a few ideas I have