Social Bookmarking Sites! A Brief Rundown

How much can these sites increase traffic to your site and bring in clients?

I’ve been doing research lately and following my Google Analytics and I’ve found that a lot, if not the majority, of the traffic comes from social bookmarking sites. It seems nowadays people really rely on these sites to find interesting content. I think it’s definitely worth it to sign up to these sites and submit your blog posts or work to these sites. It may not alway bring in a lot of visitors, but at least it increases the chance of traffic.
Which are the best sites?