I use this page to keep track of pages about me elsewhere, e.g. profiles on various community sites.
In addition to helping me keep track of these sites, linking these pages from my site helps verify that these pages were authored or edited by me. If you see a page or user profile that claims to be me but isn't linked above, or that doesn't have a link to my site, it probably wasn't made by me.
Note that having a link in one direction doesn't prove anything: I could claim to be any user online, and any user can make a link to my site. There needs to be a link in each direction to confirm that the two identities are the same person.
(and even then there may be other issues, like someone cracking into my account and changing things without permission, or community sites changing ownership over time, invalidating the links I made above.)
Twitter's Verified Account page says "One thing you can do if we haven't verified an account you're wondering about is to check the official web site of the person and see if there's a link back to their Twitter profile." — good to see this practice being encouraged by Twitter.
Google Buzz seems to be doing the right thing too, "The association between a public feed and a Buzz user relies on the Social Graph API to verify that the Buzz user is the owner of the feed. This relationship is known as a bi-directional claim, and it must be established before new external feeds are added to a Google Buzz account." — Adding new sites to Google Buzz.
Google Webmaster Authorship docs also mentions reciprocal rel=me links.
Most links on this page are labelled with XFN's me
value, so machines can make more sense out of them.
Onxiam is a site to help users keep track of their identities; my account there is ger.