Sites like Demand Studios?

There are quite a few. If you look in the SEO/Blogging section of the Freelance forum here, you'll see discussion on several places.

I heard Seed is a no go. You basically claim the title, write the content, but you're in competition with other writers. So after all that hard work, you'll likely get a rejection anyway.

I've been working for Infobarrel and Suite 101 on the side. Nothing major yet, but I'm not really pushing. As for paying up front markets, you can try Bright Hub. They pay $10 per article in addition to revenue.

A bit late to this discussion but....

Up-front - Demand, Bright Hub and WiseGEEK
Rev share/Page View - Suite101 and Associated Content
Rev share/ backlinks - Hub Pages and Squidoo

And of course, hit the job boards for private clients.

One thing I would like to add, I hear many people say, "I don't know that I am good enough to write for private clients yet." Look if you are getting articles published at Suite101, AC, Demand, Bright Hub or WiseGEEK..YOU are GOOD ENOUGH. Get out there and go after clients.

Correction..

Associated Content do pay UPFRONTS AND PAGE VIEW MONEY.