Examples would include: "Sent from my BlackBerry"; "Sent from my iPhone"; "Sent from my Mobile Phone" and so on. Often, you can control this from your mobile phone to delete it or customize it.
One Googler used his 20% time to develop a new type of Gmail signature, one that knows where the e-mail was sent from. The setting is (where else) in Gmail Labs and is a feature of the Signature tool. You'll need to click "Append Location to Signature."
According to Marco Bonechi, software engineer, "It'll use your public IP address to determine your location, so it may not always be that accurate. For example, if you're at Heathrow airport, IP detection may put you in Germany. If you want more accurate location detection, make sure your browser has a version of Gears that supports the location module. That way, Gears can make use of Wi-Fi access point signals to recognize that you're actually in London. You'll always be able to disable the option or just delete your location from specific e-mails whenever you want."
I enabled the feature this morning on my way into New York. Since I was using my wireless WAN connection, it located me in Basking Ridge, N.J. (guess which wireless company is based there). Not entirely accurate, but at least it put me in the right state.
The feature doesn't add any real usefulness, but will certainly be fun for the world traveler to add to their signature, as a way of boasting, "Look where I am!"