I'm happy to announce that Hi5 has Widget support. Yes, I know that this is soooo last year. However there's a twist that makes it better.
We worked closely with Rock You and Slide to integrate tightly with our site, using open standards wherever possible. For example, for slideshows we created Atom Feeds for each photo album, and a feed-of-albums feed for the list of all albums. And when it came time to share profile information for horoscopes (birthday) and languages spoken we used FOAF. Thus we get partners to adopt open standards, plus the work we did for them is usable by everyone.
The only tricky part was authentication and authorization. Right now it's using our own AuthToken implementation, but it could probably be done in a better way. I looked into OpenID as a mechanism, but's way too end-user centric for this type of thing.
Coming soon we should have full Atom endpoints (both in/out with WSSE auth), OpenID provider, and a few other standards based things like XMPP vCard support. All of this is being done with an Web Services Aspect Oriented toolkit called Enunciate, which has made writing these services a very enjoyable experience.