The Contacts add-on is Mozilla Labs experiment with adding contacts to the web browser.

Current Work/TODOs

Internals

Windows Contacts support
Linux support - Ubuntu first?
Make a harmonization pass between the current Contacts API and the W3C Contacts draft. Suggest changes where appropriate, and improve our API where we can. The W3C spec has a good notion of exposing Services to the caller.
Implement Groups. We probably need an indexed field on a per-person basis; it should support multivalues.
Look into database auto-vacuuming; the people.sqlite file can bloat fast.
Fix logging from resource.js

Rework the internal JSON schema. The current approach ( see https://wiki.mozilla.org/Labs/Sprints/People ) doesn't allow us to maintain attribute on a per-service basis. We would like to be able to a) indicate which service provided each piece of data, and b) throw out the data from a single service and refresh it. (See Labs/Weave/Contacts/SchemaV2)
Implement the Discoverer API as a peer to the Importer API. It should take a single person and find out more information it.

Implement "merge" and "split" in the Contacts view. Persist data into the person record to allow the merge to persist across data refresh (pending on internal JSON work)
Support multi-selection in the Contacts view (?)
Support Groups in the Contacts view.

Yahoo
Windows Live
AOL Mail
Twitter: more than 100 contacts
Generic Portable Contacts; then, PoCo-compliant sites
Generic VCard; then, appropriate sites. LinkedIn first (requires also solving captcha UX issue)
Facebook
Identi.ca
Other social networks? Orkut, hi5, Raptr, Hyves

Discoverers? May not be worth it, since we'd prefer to get this advertised to us through an identity endpoint.

Notes:

Yelp has a people search at http://www.yelp.com/member_search?action_search=Search&query=XXX - requires user to be logged in. There doesn't appear to be an API.