http://betali.st/ got us our first 300 users for http://www.marvelapp.com, it's free to submit, I recommend it, great way to get momentum.
To get our first 1000 I used a combination of Twitter, LinkedIn Groups and my own blog and newsletter
To get our first 1000 I used a combination of Twitter, LinkedIn Groups and my own blog and newsletter
I think it was around 3 months so it wasn't as fresh but what I did was go through email addresses one by one, picked out the companies that sounded the most promising and opened up a dialogue to get those guys in early.
Then kept the other guys informed to try and keep them excited. Although if I would have done it just before launching I would have saved a lot of effort.