I know how you feel. You just started your startup and all you need is the traffic to the "next big thing" you're working on. And it's so hard. So I made this huge list of all websites and directories where you can submit your startup or app and get some traffic.
It will not help your startup to success, you goal is to be featured on the app store by Apple, on Mashable, The New York Times or other websites. But you chances are very small and it takes some time. So let's start with these little opportunities right now!
I have great experience with Betali.st and other similar sites. Make your landing page nice and attract your users even before you start.
http://techcrunch.com - Don't forget, that there are not your customers, there are startuppers and investors. It's great to be there, but it will not bring you your customers.
http://mashable.com - I have a great experience with Mashable. One article about our product and 10.000 registered users in 1 day. But it's hard to get there.
I'm not talking about twitter, facebook etc. It's a must to be active there. But you should also register your startup in all the social networks for startups and investors and try to be active in your relevant communities (linkedin, quora, google+...). You can find your customers there!
If you have a great designer in your team, he should be active on Dribbble or Forrstr. This is the best place how to find if your design is good or not and also to attract some early adopters. Don't forget that this probably will not be your customers, but they can recommend you. Also, if you have great design and development, submit your website to CSS galeries like cssmania.com. It can bring you some traffic too.