PayPal has officially released PayPal X, its next generation Platform that includes a variety of APIs and other resources for developers. Primarily aimed at allowing developers to embed payment processing in a wider array of applications and environments (including mobile devices), this is a major expansion of PayPal's APIs.
The announcement was made at the PayPaxl X Innovate Conference, which gathered an enthusiastic crowd of developers and third party integrators. This was PayPal's first developer conference and it was sold out. Sebastian Rupley has a good summary of the conference over at GigaOm.
Additional documentation on the PayPal X APIs is available, including documentation on the now publicly available Adaptive Payments API. PayPal has also released several client libraries, including PHP, ASP.Net, and Java libraries. A gallery of sample apps is also available to get some additional insight into how the APIs can be used (a PayPal developer account is required).
To promote adoption of the PayPal X APIs, PayPal has also launched a Developer Challenge with several cash prizes, including a $50,000 first prize. If you're interested, you'll need to enter the contest by December 15, 2009, and your application must be submitted as a live app and demo by February 1, 2010.