Paypal-10 is a project mainly written in PHP, it's free.
Paypal Module (key-value pair) for Kohana 3
Adding more stuff soon.
PayPal NVP https://www.x.com/community/ppx/wpp https://www.x.com/docs/DOC-1374
IPN (Instant Payment Notification) https://cms.paypal.com/us/cgi-bin/?cmd=_render-content&content_ID=developer/e_howto_html_IPNandPDTVariables https://www.x.com/docs/DOC-1084 https://www.x.com/docs/DOC-1082