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.