From what I’ve observed, most of the “programs” you’ve listed are networks, and most of them support dozens, hundreds, even thousands of merchants – in a huge variety of niches. Amazon is not technically a network, unless you factor in the presence of about a dozen entities like Zappos, Woot, Endless and the like. With very few exceptions, networks are diversified. Performance-Based.com focuses on eco and green merchants. Some support a particular locale – European merchants, for example.
A good alternative to Amazon Affiliates is the Target Affiliate Program. They are another big business, who sells a huge variety of product, so it’s very easy for bloggers to fit links easily into their content. The downside is that Target is a popular shopping place, so affiliates may miss out on potential commissions when people choose to shop in-store instead. Nonetheless, it is still a great affiliate program.
In 2006, the most active sectors for affiliate marketing were the adult gambling, retail industries and file-sharing services.:149–150 The three sectors expected to experience the greatest growth are the mobile phone, finance, and travel sectors. Soon after these sectors came the entertainment (particularly gaming) and Internet-related services (particularly broadband) sectors. Also several of the affiliate solution providers expect to see increased interest from business-to-business marketers and advertisers in using affiliate marketing as part of their mix.:149–150