I find there are two extensions that I install for Thunderbird every time I set up a new computer. The first is Plaxo for Thunderbird, which does a pretty good job at syncing contacts. The second is Messagefaces with Gravatar support turned on, which you can find in Add-ons > Options > Online Services. (Wish it was on by default, it doesn’t really work without it.) Plaxo has an avatar feature but I turn it off because way more people have Gravatars than Plaxo avatars.
It recently became more important for me to sync my address book across several computers on various platforms. Solutions like LDAP seemed like a pain and had bad support in Thunderbird. I don’t want to go to a hosted app like Joyent or Zimbra, and I need to be able to work offline. Anyway in my searches I came across Plaxo. In the past I grew to hate the Plaxo contact update spam I used to get every day, so I had pretty much permanently written it off.
However this time when I saw they had support for Thunderbird, Mac OS X address book, and Yahoo and I got pretty excited. I tried it out, and I am now syncing a Mac Mini, a Powerbook, a Macbook, my Windows desktop, and a Vaio laptop to a single address book. It cleaned up dupes pretty well, and the online interface is surprisingly usable as well. This is also the best way I know of to get Thunderbird to use the OS X address book, so you get integration with all the other apps like Adium which feed off that.
What could be improved? Sync is really hard, and few do it well. My experience with Plaxo has been pretty good thus far—I think I’ve avoided spamming anyone for contact updates—and I’d love to connect other bits and pieces into the Plaxo cloud. They should open up their API so developers can start to integrate the system into other products and services, and it can become a de facto standard.
Update: They do have an API, I had just missed it. Cool!