Betalist (which others mentioned) seems to deliver at least 300ish signups for a generally useful service. Unfortunately my latest project is targeting only Heroku users who need to install SSL [1] so I needed to do something else.<p>The best thing I found was actually Twitter. I setup a search column in Tweetdeck that would pop an alert on my desktop if anyone tweeted "Heroku" and "SSL" in the same tweet. I'd then just @message them and ask if they'd want to try the alpha (Heroku has strict phases with increasing numbers of users you need to onboard before they'll release the add-on to General Availability).<p>If someone agreed to be an early tester, I'd try to "upsell" them into taking the time to do a Skype onboarding call with me where I'd just watch them in real time try and add a SSL cert to their app. This was likely the fastest and most productive thing I've ever done to rapidly improve the product, hugely benefical.<p>1 - <a href="https://addons.heroku.com/expeditedssl" rel="nofollow">https://addons.heroku.com/expeditedssl</a>
We allocated a small marketing budget for the following:<p>1.) Offer promo codes to increase conversions<p>2.) Hire a writer and get quality content<p>3.) Provided incentive for users with a referral<p>4.) Email marketing
We tried a bunch of methods, getting press, cold calling, newsletters, blogs, social media, etc. We learned that the most important thing is knowing who your target first customers persona should be and why (e.g. nicolasd, in another comment here, identified their Venn diagram). The more that you can narrow and define that persona, the easier it will be for you to learn about their habits and then get exposed at those places and speak to their needs.
Last year we started a project (didn't work out) but we had a quit cool method to get emails and sign ups. In our case, we used the data of 500px - you can read about it here: <a href="http://saloon.io/geek-approach-to-marketing/" rel="nofollow">http://saloon.io/geek-approach-to-marketing/</a><p>Even if you can't use the same strategy, maybe it gives you an idea for your customer field :)
Betalist, handful of beta and nonbeta startup directories, reddit and HN.<p>If its a good product that will get you +250 users (we got about 330 while we were still in private beta).<p>Beyond that we sank a couple hundred in ads at a $0.2 CPA to gain our beta base. Then a month later we got placement on techcrunch, venture beat and a few other major outlets while still in private beta