GMail contacts and Thunderbird
This works quite well, but it only synchronises with other Thunderbird sessions. What I really wanted was to use the Contacts from GMail and synchronise with that. That way, whether I'm using Thunderbird, or just the GMail web interface, I'll have the same details available for all the people in my contacts list. The problem is that Google hadn't provided an API for accessing the Contacts data, but that all changed last month.
The good news is that Zindus are already working on a new extension for Thunderbird to synchronise with Google Contacts. I plan to try this out very soon.