Tagalag
Tagalag, by John Wehr, has recently opened its doors. It is a contact manager that allows you to assign tags to email addresses and view tags that have been assigned to you. John is a close personal friend of mine, and he helped us out a bit with MochiBot and MochiKit this summer. Tagalag is cool stuff, and it's free, so go check it out!
It's using MochiKit for the UI, and has a lot of cool features such as GMail and Yahoo! Mail integration with Greasemonkey, a Tagalag box you can throw on your blog or website to make it especially easy for people to tag you, and a Google Maps view of identities in the system that have provided zip codes.
Interesting to software developers is that it supports the XFN vocabulary, and has an XML based API. Locally, all of the data travels over JSON before it hits the MochiKit-laden JavaScript code.