There are so many jobs listed for developers familiar with the PayPal API on Freelancer.com that the site has a category dedicated to it. And that's not all--there are over 300 API-related listings on the site, which connects freelancers with people who need programmers and designers.
The popularity of PayPal's API is understandable. It serves a core ecommerce Function, it's a brand name, it was one of the earliest APIs on the web, and they've recently had a major upgrade. Those who post the projects are charging money for something. A good Integration with PayPal means dollars, so they're willing to pay.
Of course, they aren't willing to pay very much. 80% of all projects have a budget below $750. That's par for the course with these project posting sites. Still, it's interesting to see what API-related projects are posted.
As with mashups we list on Programmable Web, projects incorporating maps are common. Twitter, YouTube and Facebook are also popular. Google Wave already has a category of its own and a few projects.
There is no doubt a need for developers who can work with APIs. And the demand is likely to increase. Hopefully as demand increases so will the budgets.