The Gmail team routinely adds features to its service, but it doesn't roll out many of them to users without explicit permission.
To see and enable the list of features in Google Labs, log into your account and click the Gear icon in the top right corner of Gmail. Click labs.
From there, navigate to the Labs heading to a list of fun features you can try out. I use Background Send a lot.
If you don't see the menus above, click the gear icon in the top right corner and click Mail Settings. Then select the Labs tab to explore Gmail Labs.