Amazon has cemented itself as a top tier option for charging users thanks to its Amazon Flexible Payments Service API. The crowd-funding site Kickstarter has run hundreds of millions through the API, so it should also get some credit. Although, at Amazon, Kickstarter may get blame instead, having inspired many other sites, which eventually caused Amazon to turn away sites with similar concepts.
Read-it-later service Instapaper has released what it's calling a "full API." With it, developers using the Instapaper API can create applications that both write to and read from the service. The full API is free to developers, but only paying users can read from their Instapaper account using third-party tools.