Payment APIs That I Am Watching

I'm taking a look at the world of payment APIs right now. As with all my other monitoring of the API space, I am only looking for the best approaches, by the most interesting companies in the space--I don't have time to track on everything, 

I am looking to take a snapshot of the payment API space, understand who the key players are, and how they are delivering valuable payment API resources that developers are actually using. So far I am tracking on 34 separate payment APis: