I added XFN support to Workbench this morning, putting keywords like "friend", "acquaintance", "met", and "colleague" inside the links on my blogroll. This tells XFN-aware software how I know people, which can be used to build distributed MySpace-style social networks across the web.
I'm a generation too old for this kind of thing to seem normal, but I'm curious to see whether anyone's doing interesting stuff with this data.
WordPress MU also supports XFN. Any time you add a link to your blogroll, click the Link Relationship pane to expand it and define how you know the linked site's author:
My newly codified relationships should be picked up soon by RubHub, an XFN search engine with a disturbing name. I do not plan to rub anyone on my blogroll, no matter how close we seem in this site's markup.