Rated 5 out of 5 stars
This is so useful and well integrated into Gmail that you don't feel like you're using a separate product at all. It's a genius use for the right hand side of the page.
At it's simplest it lets you put a face to an email address. But as well as giving you contextual information it allows you to take actions as well: like follow someone on Twitter, or connect LinkedIn, after you've exchanged emails with them.