I had the great pleasure yesterday of attending a great presentation by Praveen Alavilli (@ppalavilli) on the new Force.com for PayPal X Adaptive Payments Platform toolkit. PayPal has taken on the work of making a set of Apex Classes and Custom objects available for developers to use to add commerce activities to any application built on Force.com.
I had a chance to take the toolkit for a test drive last week. Once you have setup your developer account on PayPal and have obtained a Force.com Developer Edition, you can begin simulating various payment processes in about 10 minutes time.
If you don't know much about the PayPal Adaptive Payment Platform you can visit their API Page to learn more. Before I took a look at their offering, I wasn't even aware that there are some many different ways that a merchant, selling partner, landlord, property management company, or even parents and grandparents might need to distribute funds!
PayPal X is the developer site for PayPal. It is essentially PayPal's Developerforce! To get started you will need to sign up for a PayPal account (at no cost), join the PayPal developer program and then create a PayPal sandbox so that you can experiment without real money changing hands.