PayPal is offering a new open source toolkit for Google App Engine that lets you integrate Java apps running on App Engine with the new PayPal Adaptive Payments API. The Adaptive Payments API lets developers embed payments into their apps, services, and platforms.
PayPal's Praveen Alavilli says, "The Adaptive Payments APIs offer several new payments functionality like Split Payments and Preapprovals, that enable developers to implement a variety of monetization models - freemium, subscriptions, pay-per-use, value-added-services, micro-transactions, e-commerce, etc. in their applications built and running on the App Engine. Whether you are building an application for Businesses to process back-end disbursements or payouts to affiliates, or building a social or gaming app for Facebook / Twitter / Open Social, or building a Desktop gadget for premium content, or building a Geolocation app that only helps users find places and people around where they are but even help in transacting them, or building a Web2.0 AJAX app that mashes up content and services, or several more use cases enabled by App Engine, now you can use the PayPal X toolkit to enable payments in them as it fits the needs."
More information on using the toolkit can be found here. There will be a Python version of the toolkit coming out soon, PayPal says.
On a related note, PayPal is reportedly launching Mobile Express Checkout, a mobile version of its existing Express Checkout service.