I have found this example :
http://www.magentocommerce.com/magento-connect/log-in-with-paypal-access.html
Does anyone know where can I exactly find the API for this? I can't find such information how to integrate Login-via-paypal on any paypal pages.
Its Paypal Access. Try this links to know about paypal access and to integrate Paypal Access in your site. https://www.x.com/developers/paypal/products/paypal-access https://www.x.com/developers/paypal/documentation-tools/attributes https://www.x.com/developers/paypal/documentation-tools/getting-to-know-paypal-access