The Salesforce.com/PayPal integration project
This is the open source code base for a Salesforce.com > Paypal Masspay integration, as presented by Simon Lawrence at Dreamforce, and as found in the blogposts:
http://srlawr.blogspot.co.uk/2012/01/integrating-paypal-api-with.html
and
http://srlawr.blogspot.co.uk/2012/02/processing-paypal-ipn-messages-with.html
Please read the blog posts for how this code works, and how to get it up an running from within Salesforce. If you have any questions or feedback, please get commenting on the blog, or give me a tweet @srlawr.