This is a million dollar question and everyone who runs an Internet marketing / SEO services agency is looking for an answer to this question.
Here are my 2 cents:
1) Site Auditor (Standalone tool): This could be a true lead magnet for digital marketing firms.
Have a free site auditor tool embedded on your site and let people audit their site by entering their site URL. To provide them the full website audit report, you ask for their name and email Id.
By looking at their site audit reports, you know where they need improvements. You can create a customized service package based on that and can offer them in a follow-up mail.
You have a high chance of conversion in comparison to an email blast wherein you send out a mail just talking about what you offer (and do not know anything about what they need)
2) Mobile Prospector: Approaching businesses who don't have sites and convincing them to have one can be very tough.
A comparatively easier task would be --> Finding out non-mobile friendly websites and reaching out to their owners/ representatives. You can offer them customized packages to make their sites mobile friendly. (Again giving something that they lack and not just talking about what you can offer)
The icing on the cake is, there are tools available that not just tell you about the mobile friendliness status on a website but also scraps & provide you the contact details. You just need to reach out to them with your customized offerings