iphigenie drew my attention to
Xobni several weeks ago in this
post:
One add on I have been testing at the moment is xobni, which mostly analyses contacts and emails to produce nice little profiles, for each contact, showing not only the history of interactions (and the wonderful "files exchanged with x" section listing all attachments sent or received, which means I only need to find an email from that person to find any attachment they sent me, i like that!) but also social features like who they are connected to, emailing patterns, and some quick overview tools
Sounded interesting so I googled it and applied to join the beta programme. My e-mail inviting me to the beta party came in yesterday and I am now using it. I'm interested in other users' experiences with it so am starting this thread.
If you are intested in trying it out, I have 5 invites to give out. Let me know if you are interested (it took me several weeks to get a spot through signing up at the website) via PM or e-mail and I'll send you an invite. First five across my inbox will be served.
From my limited experence with it so far, I'd defintiely like to see more user customization allowed - particularly colour scheme and indexing.

EDIT: switched to inline thumb attachment...