I've been using their article marketing service to great effect.
They've put $8,000+ in prizes up for grabs for the people who can add the most sites to their network. It doesn't cost a thing to join and add sites.
The only requirement seems to be that a site must have an XML-RPC remote publishing API, such as the built-in API of WordPress.
If you join for a free account now, you also get a 7-day free trial of the paid section of their service.
Their URL is Niche Article Marketing for High Search Ranking - Premier Service (non-aff link).
Read about the Site Smackdown Competition here:
AMA Site Smackdown Competition - Over $8000 in Prizes