Those who use GMail regularly be familiar with the Google Talk sidebar showing which of your friends are online, as well as the wide variety of customization options available. A new one fresh from the labs is Android icons that let you know if your contact is currently online thanks to an Android device, complete with yellow Away robots, and red Busy ones. This would be a great option even if it were just extra eye candy for the Android-obsessed, but knowing someone is mobile definitely helps contextualize the conversation, and potentially avoids anyone getting offended at short, terse replies. You can enable the option in GMail over here.
[via Official GMail Blog]