Firefox/Chrome: Gmail's address book is nice, but isn't mind-blowing. Free extension Xobni automatically grabs info about your contacts from the web and puts tons of information right in Gmail's sidebar, including relationship history, mutual contacts, and even a photo.
We've loved Xobni's Outlook plugin for a long time, and we're super excited to see it finally appear for Gmail, our favourite mail client. Xobni pulls information about everyone you email from message bodies, the web, and social networks to give you more information about them—even if they aren't on your contacts list.
After installing Xobni and opening up a message in Gmail, you'll see the ads on the right side have been replaced with a new contact sidebar, showing a picture of your contact, their email addresses, and even phone numbers if they've listed them in email signatures or other messages. You can also view other recent messages sent between you two, a relationship history, and other social updates if you link your Facebook, Twitter and LinkedIn account with Xobni. Basically, it takes the ad space on the right side of Gmail and makes it really useful, without you having to even lift a finger.
Xobni for Gmail is currently in private beta, but Lifehacker readers get in on it a bit early. To check it out, just sign up at the Xobni for Gmail page with the code XOBNI-LIFE. They're still getting everything up and running, so you may not get in right away, but you should get in within the next day or so.
In addition, Xobni's also working on an app for iPhone and Android. It's currently in the Alpha stages, but the XOBNI-LIFE code should work for it too. You won't get an invite right away (since the apha's closed at the moment), but you will get first priority when they open it back up again—so head over to the iPhone and Android pages to check that out as well.