I did not tested yet, but it seems very usefull;
Gmail Notes: "Gmail Notes is a web application that adds annotations to Gmail. With Gmail Notes, you can create a private note for each conversation in your Gmail account. You can make a note to, for example, summarize the key points in a long conversation thread. Or to outline some thoughts for a follow up email or phone call. For a list of more ways to use Gmail Notes, see our Typical Uses page."