MailBrowser is a new Gmail 3rd-party browser plugin that sits in the sidebar, giving you more information about your Gmail contacts. The plugin works by downloading a complete local copy of your email archive from Gmail over IMAP, then cross-referencing it to your Google Contacts in your Google account. I should state up front that the MailBrowser plugin is in beta, and likely to change.
MailBrowser for Gmail presents useful information about the contact you are viewing like a list of your last conversations with them, trending information about your communications with the contact, and fields so that you can add tags and notes to the contact in a very similar fashion to Xobni. It also attempts to look up the domain name of the contact and from there grabs information from the domain's website, which often turn out to be meta tags that are not all that useful. It would be nice if MailBrowser allowed you to choose the order it shows this information, or to hide sections entirely, because the most important information is hidden down at the bottom.
Beware: because (as mentioned above) MailBrowser works by downloading a copy of all of your emails, it will use up a significant amount of disk space.
It will probably take the plugin a few hours or even days to index your entire account, depending on how much email you have.