Both Digg and Reddit can do mind blowing wonders for your site if you make the front page. HackerNews, a similar site, doesn't have as much traffic but it does have a more intellectual user base that can provide critical feedback on your site. However, these days hitting the front page is getting harder and harder due to the large amount of submissions from a rising number of sites. Really great content will manage to find it's way to the front page at some point or the other, so don't get discouraged if your initial submissions gets buried.