'Reply by Chat'

Reply by chat - one more feature being added to GMail. It lets you to discuss an email on the chat box. When u got an email, instead of sending reply to him, you can chat with the user when that person is online. The discussed mail will be saved in your chat history ( for that u need to enable save chat history, which is by default). Chat histories are only grouped with conversations when you use the Reply by chat feature. If you close a chat window after you discuss an email with someone, and then initiate a new chat, your chat history won't be grouped with a conversation.

A nice feature and integrating both chat and mail to keep up the context.