Hey I was playing around the OpenSocial and I prepared following sample application।Have a look at it we are coming with many opensocial apps.
This is just a plain iframe application so nothing much to brag about but still it is an opensocial application 
Anyways we are working on the platform and will be coming up with many more exciting applications very soon।
To develop application on Opensocial you will require Sandbox account of Orkut (if you want to build for Orkut).You can get that from http://sandbox.orkut.com/
Also you will require little understanding google gadget development.
Check out the opensocial APIs here।
Happy Coding,
Kuntal