Reviewr “ties into the API exposed by Ludicorp’s […] new social software application, Flickr and hooks it up to the API exposed by Amazon. The point is that using Reviewr allows you to search for reviews of products by people you know and trust.” (via Hublog)
Interestingly, as I was proposing in a previous post, Friendr limits the number of contacts an user can have. It was not a totally dumb idea after all…
Check the services already created using the API and the services documentation (1, 2)