ClickBank allows you to join for free, and the approval process is virtually automatic, so it’s a great choice for people entering the affiliated game for the first time. ClickBank has a ton of information, including FAQs, walk-throughs, and videos available, so the barrier to entry is quite low. There’s also a (paid) program called ClickBank University with courses and assistance from experienced marketers.
JVZoo was founded in 2011 and has since rocketed to near the top as one of the most popular affiliate programs out there. JVZoo is unusual in that there are no upfront costs for either publishers or merchants (advertisers). JVZoo’s income is exclusively from charging fees (to both the merchant and the affiliate) after a sale has been made. It is also unusual in that it pays commissions “instantly” via PayPal rather than once a week/fortnight/month like other affiliate programs.
While these models have diminished in mature e-commerce and online advertising markets they are still prevalent in some more nascent industries. China is one example where Affiliate Marketing does not overtly resemble the same model in the West. With many affiliates being paid a flat "Cost Per Day" with some networks offering Cost Per Click or CPM.
SkimLinks is probably best for bloggers who want to write content around the affiliate link rather than add affiliate links to existing products. SkimLinks offers a lot of tools to compare commission rates and offers in order to customize your content to optimize your income. Once nice aspect of SkimLinks is that it offers lots of products for non-US creators, including popular UK brands like John Lewis and Tesco.