Gmail Conversations love them
One of the prime reasons I moved from yahoo mail to gmail was the conversations feature provided by Gmail. You can see this feature in all glory in the following image. What it shows is the response to my Happy Diwali email I sent to few of my friends
In the non-conversation world I might have ended with that many emails and it would have been so not-cool to manage them. Here on Gmail I have them tied in a nice bundle.