OpenSocial
Google’s OpenSocial looks promising.
I’ve been fairly enthusiastic about the way FaceBook have opened up their site to 3rd party applications, but at the same time it’s worried me that they’d monopolise everything eventually.
An open API is a much better idea. I have to find out more about it to see where the potential pitfalls are, but I definitely think it’ll be worthwhile developing appications for OpenSocial.