It happens to be the product that inspired us when we started building RTM back in 2004 -- that's right, the one and only Gmail. We've always thought it would be incredibly cool if you could manage your tasks alongside your mail -- and have your tasks know what's on your calendar and who your contacts are too.
Remember The Milk has a simple interface to provide task management in Google Calendar, have an iGoogle Gadget, send reminders via Google Talk, show task locations with Google Maps, and provide offline access with Google Gears. [RTM Blog]
Remember The Milk for Gmail is a Firefox extension that that allows you to manage your tasks in Gmail (complete, postpone, and edit tasks), add new tasks (and connect them with your emails, contacts, and Google Calendar events), automatically add tasks for starred messages or specific labels, and much more!" So you can add events to Calendar directly from Gmail, you can flag messages for follow-up and see which tasks are connected with a certain contact. You can also create tasks every time you star or label a message and let RTM to auto-complete tasks using names from your contact list or Google Calendar events.
The Milk fits great in the landscape by using visual elements from Gmail Chat, Google Reader, by using even small features from Gmail (like undo) and creating a section for tasks in the settings. This is definitely the service that has the best integration with Google services and it looks so good in Gmail that it should be a part of Google's mail application.