The following list of 5 pay-per-post companies connect bloggers with advertisers. Basically, advertisers pay bloggers to write about their product. Compensation is based on a blog's "reach". Traffic is a big part of the equation, but your blog's design, focus, community engagement, etc. are also used as metrics for determining how much you'll get paid per post.
The terms "pay-per-post" and "sponsored post" are interchangeable.
1. PayPerPost
PayPerPost is a pioneer in the sponsored post industry. As a result, they have the largest network of "Posties" (what they call their blogging publishers) and paying advertisers. There are extensive tutorials, a well established code of conduct for bloggers, and a large community forum for getting help from other Posties.
PayPerPost is broad but "shallow". The bloggers here are generally smaller, but there is a lot of them. Same with the advertisers. This means there is plenty of support from fellow bloggers, making PPP a great opportunity for small blogs who are just starting out. Larger blogs, however, may get lost in the shuffle and might do better at smaller marketplaces where they can really stand out (see ReviewMe below).
2. ReviewMe
ReviewMe is the other major player in this industry. After PayPerPost and ReviewMe, there is a considerable dropoff in the size of the remaining sponsored post marketplaces.
Based on browsing the marketplace and reading other reviewers, it seems like ReviewMe has (generally speaking) fewer but larger publishers, leading to higher average rates. ReviewMe says bloggers can expect $20 to $200 per post (after their commission).
ReviewMe only accepts blogs that meet a minimum number of inbound links, feed subscribers, and traffic.
3. SponsoredReview
SponsoredReview lets bloggers and sponsors directly negotiate rates. Bloggers can also bid on existing sponsor offers.
Their direct negotiation, or "two-way system" as they call it, means you have to browse through the advertiser directory and seek out those you want to work with. Advertisers can do the same, and have to make individual offers directly to bloggers.
The commission rate is 35%, one of the lowest in the industry. Payouts range "anywhere from $10 to $1000 per review". Payments are made every two weeks via PayPal.
4. Blogsvertise
Blogvertise is interesting because there is no marketplace to browse. The staff connects the advertisers and bloggers they think are a good match.
You register for an account with your blog, and when Blogsvertise has a suitable "task" for your blog, they will email you the offer.
According to their FAQ, the average blogger payout is $5 - $15 per post. You are paid 30 days after your sponsored post is approved by the advertiser. Payments are made exclusively through PayPal.
5. Smorty
Blogs must be at least 30 days old and have at least 10 pieces of content.
Smorty's quoted rates for each sponsored post ranges from $6 - $100 to the blogger. Your blog's PageRank and advertiser ratings of your previous sponsored posts determine the frequency and value of offers you get. Payments are made weekly, exclusively through PayPal.