Here are the Alexa rankings for some of the sites I've looked into:
EzineArticles.com - 255
Scribd.com - 265
Ehow.com - 476
AssociatedContent.com - 773
SlideShare.net - 999
Suite101.com - 1,669
ArticlesBase.com - 1,826
Today.com - 2,139
Helium.com - 3,987
Newsvine.com - 6,555
Constant-Content.com - 40,541
Triond.com - 41,397
Bukisa.com - 53,810
DailyArticle.com - 234,725
The traffic volume is not the only factor to consider however. For example Bukisa is currently paying writers around $4 per thousand page views, while Associated Content is paying only $1.50. So while your content may get more hits on AC, you might still make more money on Bukisa.
Some sites are also growing much faster than others. Bukisa is still in beta, and might grow very quickly in the future - or it might cease to exist altogether. Scribd is another young company, but already pulling in huge numbers of visitors.
Some of the sites, including EzineArticles, don't pay at all. However, they could be useful in building a reputation for yourself, showcasing your work, and directing people to your website. These sites may not pay cash, but they could be said to pay you in links - a currency that buys you Google's attention.
Finally Constant Content is a marketplace to sell work. So rather than making fractions of a cent for each reader, you are looking for someone to buy your work for a one off payment ranging from $10 to $100 or more.
EDIT: Read the comments for a view from an experience freelancer about these sites.