I/O Docs enables developers to experiment with API calls from within the documentation. You can see it in action at
Posterous and Wordnik Alibris and Klout (correction: the Posterous and Wordnik APIs were the inspiration for I/O Docs, but don't use I/O Docs).
Accoring to Mashery's announcement: " In surveys that Mashery conducted at SXSW and other events, developers consistently cited poor API documentation as a major obstacle to getting started with an API." Cutting and pasting keys is also cited as an obstacle, so I/O Docs input keys automatically for logged-in developers.
Disclosure: Mashery is a ReadWriteWeb sponsor, as is Programmable Web's parent company Alcatel-Lucent.