Several months ago I tried out Etacts Gmail Plugin. At that time it just didn't seem to work for me at all.
Then recently - prompted by an email from Etacts - I tried it again. And I have to say I'm really liking it! Whenever I look at an email, relevant information about the person the email is from appears in the sidebar based on what Etacts can pick up from my social networks.
It even picks up publicly available info about people I'm not directly linked to and so suddenly I have a lot more info at my fingertips when I'm talking to people I may not know that well.
A useful additional tool!