I am working on an old site of mine, and it has an old "Google Fusion" button. I don't remember what it did, but I think it allowed people to bookmark the RSS feed in their account? The button no longer works (just goes to the Google homepage). I have been trying to find some information about the new protocol for allowing people to bookmark a page in their Google account by clicking a button on your site (or is this completely obsolete now?).
I know this seems very elementary, but it's not something I've kept up with.
fusion.google.com was used as a domain for scripts to add for example sites to people's iGoogle or Google Reader environment. As both iGoogle and Google Reader are now discontinued products, there is not much use anymore of both the fusion.google.com domain and the Google Fusion button.