The PayPal SDK
PayPal has very quickly become a standard for internet payments especially since being bought out byEBay. Not only have they gone mobile, but the completeness of the SDK offering is very good. Windows is supported under either Java, Cold Fushion, ASP.NET, Classic ASP or PHP. Linux and Solaris are supported under Java, Cold Fushion and PHP. It all starts at the Integration Center which can be found here:
LaterC