I don’t know how to express the mild disappointment of finding out that the OpenSocial API, despite its alluring name, is anything but. Why couldn’t they name it the OpenWidget API or UniversalSocialWidget API or something less misleading. OpenSocial sound so much like a common API interface for web services that third-party developers can take advantage of. Instead you have a bunch of common API calls so developers can build portable widgets for all the participating social networks. Of course, the catch is you have to do it all with javascript and html.
I guess the main benefit would be for smaller social websites getting accepted into OpenSocial and using that to gain a foothold from other participating apps, since then they’ll have internal access to data from other social apps.
You have to write your own website to deliver Javascript and HTML too; thats what browsers run. What OpenSocial adds is the ability to avoid writing user and friend registration code, and getting access to the registered users of other (probably larger) sites.
You can call back to your own server-side code from OpenSocial, so you can develop your web application in whatever language you want.