FOAF is an RDF language describing social networks and related information. FOAF tries to social networking available to the Semantic Web, making your identity and friendships machine readable. An interesting FOAF application, if you have enough tech-savy friends is Foaf+SSL. For an intuitive explanation of how FOAF+SSL works, take a look at Henry Story's paradigm shift post.
For the curious, I've created my own FOAF file. You should also check out the SIOC Project's Semantic Radar plugin, which notifies you of the existence of interesting RDF data as you browse the web (and inspired my rel-vcs plugin).
The foaf:mbox_sha1sum property can be generated with:
$ echo -n 'mailto:firstname.lastname@example.org' | sha1sum