FOAF-a-matic is a simple Javascript application that allows you to create a FOAF ("Friend-of-A-Friend") description of yourself. You can read more about FOAF in Edd Dumbill's "XML Watch: Finding friends with XML and RDF" article, at the FOAF homepage on RDFWeb, and also the FOAF vocabulary description.
In short though, FOAF is a way to describe yourself -- your name, email address, and the people you're friends with -- using XML and RDF. This allows software to process these descriptions, perhaps as part of an automated search engine, to discover information about your and the communities of which you're a member. FOAF has the potential to drive many new interesting developments in online communities. Ben Hammersely's "Click to the Clique" article for the Guardian Unlimited website further explores these ideas.