One of the best APIs we have had the pleasure of using belongs to Campaign Monitor. Campaign Monitor (CM) is a web service where clients can manage their email lists. We have developed applications that bridge our own client’s applications with services provided by CM. The service is top notch and their API is a joy to work with.

We’ve also worked with PayPal’s API. PayPal offers a wealth of developer documentation at x.com. Unfortunately, the documentation is inconsistent as you travel between the official documents, the online documents, the support forums and the unofficial forums (where many of the answers are found of course). I posted an article to specifically address some of the hurdles you will encounter in configuring Recurring Payments (subscriptions) for PayPal’s Website Payments Pro plan.

Companies offering their own APIs would do well following Campaign Monitor’s example. Solid code and solid documentation make them an easy referral for future clients.