How can you gain legitimate email sign ups? The normal method would be to include a sign up option at in multiple locations on your business website. For instance, your main page should have a mailing list opt in, as should any registration forms, the signature of any emails you exchange with existing customers, even your terms of service page may mailing list discreetly include this opt-in link. You should always 'sell' this option. Offer a reason why visitors would want to be on your list.
This could be an offer of periodic discounts or news regarding the latest products you have in stock. You might even offer an instant mailing list discount to encourage new sign ups. You should also have a sign up field that allows members to decide what format they prefer the emails. This would be either plain text or html. What can you do with your mailing list once you have compiled it?
Anything you want - within reason. However, if you abuse your list you are likely to annoy your customers and risk having them opt-out of the list. Common uses for a list would be to advise customers of an offer or discount you are running. Sales and mailing list clearance items may also be announced in this manner, however it is advisable to do so in one email to cover a broad range of issues.