Latest In Gmail labs You’ll love it

YouTube, Picasa, Flickr, and Yelp previews
Instead of just links, see previews of photos, videos, and reviews right in your email.

Undo send well this functionality is really the best which i love among all it happens many a times that you forget to mention something or you typed wrong now.Give yourself a grace period of a few seconds to cancel sending, then edit your message before sending again.

Tasks mobile
Take your to-do list everywhere you go. Just go to gmail.com/tasks from your mobile browser.

Offline
Make Gmail work even when you’re not connected to the internet.

How to start these all features

Simple go to gmail enter in it and go to settings tab and then to labs and enable the functionalites which you want.