Great question, with a million different ways to get it done, but none without a lot advertising/word of mouth in some way, shape or form. I remember at one of my startups, we grew 350k+ subscribers in a couple of months, but it was very painful and expensive. Today, you're in luck. There is a new tool you might try instead - Kickstarter. Post a campaign on Kickstarter: raise some much needed capital without giving away equity, sign up some very interested people to the next version of your productivity tool and get immediate market feedback. If you run the campaign for 30 days and get more than 5000 people, which will all be highly motivated evangelists, to sign up...you're a double winner. You will have met your goal and gotten additional funding. If you don't, it's great marketing feedback that something is not quite right yet with the product or the value proposition. I would be happy to provide additional thoughts on how to do this with a follow on call.