Another feature that we worked on this week was integrating the "standard" version of PayPal as an additional payment option for CiviContribute. We waited for quite a few months for PayPal (or Google) to release a nice web 2.0 api for subscriptions / recurring transactions, but that did not seem to be happening. So we took the plunge and decided to implement PayPal Web Standards as the first step towards implementing PayPal Subscriptions (basically recurring donations)
PayPal has done an incredible job of documenting their API's and making the integration fairly easy. It helped that the Drupal e-commerce package has this feature, so we could peek into their code base and figure out how to do a few things. Getting a system up and working using the sandbox makes development so much easier, since the sandbox behaves identically to the live system. I was pleasantly surprised to see that we did not have to write a lot of code for this case, but primarily had to rearrange and cleanup the existing code to handle the synchronous (SOAP api) vs the asynchronous (IPN) implementation.