1) Watch Youtube Videos In Gmail Chat

With this feature you can watch previews of YouTube videos whenever you receive a YouTube link in an email.

It looks like this:

To enable it go to gmail labs , search for YouTube previews in mail and select Enable and click on Save Changes at the bottom.

2) Use Gmail Offline With Google Gears

It is a web browser plug-in , that allows you to work in Web applications without internet connection. When you lose your connection, Gmail automatically switches to offline mode, and uses the data stored on your computer’s hard drive instead of the information sent across the network. You can read messages, star and label them, and do all of the things you’re used to doing while reading your webmail online. Here you can find tutorial guide step by step “How to install Google Gear “

3) Undo Your Sent Message

Turn on Undo Send in Gmail Labs under Settings, and you’ll see a new “Undo” link on every sent mail confirmation. Click “Undo,” and we’ll grab the message before it’s sent and take you right back to compose.

4)Google Calendar gadget

This gadget is a simple way to see your Google Calendar agenda and get an alert when you have a meeting.

5) Navbar Drag and Drop

With this feature you can re-order your navigation as you like.