Gmail is one of the most popular free Email service from Google. Gmail has a feature call Gmail labs, using it you can add many new features to your Gmail account. You can access Gmail lab under Setting -> Labs.
If you will enable at least one of the lab then a shortcut will create at the top right corner as shown below:
As if Gmail wasn’t powerful enough, you can find all sorts of goodies and extra features in Gmail Labs. The list is pretty massive, so I’ve narrowed down to 10 favourite labs to help increase your email productivity.
1. Multiple Inboxes This is one of our favorite Gmail features —there’s just so much you can do with it. Multiple Inboxes has you covered by letting you stack extra lists of emails—whether they are labels, starred messages, or any other Gmail search—in your main inbox window. There’s no limit to what you can do with Multiple Inboxes, so if you’ve ever felt anything was missing from Gmail’s inbox organization, you can make it yourself here. Note that, unfortunately, Multiple Inboxes does not currently work with Gmail’s Priority Inbox feature, so you’ll have to decide which is more important to you for a particular account.
2. Message Sneak Peek The Message Sneak Peek feature is simple, but pretty useful—instead of opening up every message you get, you can just mouse over them(or right Click) to see what’s inside a message. That way, you don’t have to go through to trouble of opening it, marking it as unread, or filtering it if you can’t deal with it right away. 
3. Inserting Images The Inserting Images lab will help you out by letting you insert images easily—whether you upload them from your computer or link to an external source on the net—with the press of a button. Note that if you’re running Google Chrome or Safari, you don’t need this lab enabled—you can just drag and drop images right inot the Gmail.
4. Undo Send This is a great feature. Just imagine this… you have to mail someone an important email and you type the email with some big typos in it. When you just begin to read your email you accidentally click the “Send” button and then you realize that there were some typos. With this labs feature you can cancel the email which is being sent or if it has been sent, you will have 30 seconds to undo this and then you can edit the email and send it with relief. 
5. Default ‘Reply to All’ When multiple people are involved in an email thread, one person will break off by accidentally hitting the “Reply” button instead of “Reply All”, and then everyone else misses that part of the conversation. Save yourself from being “that guy” by enabling the Default Reply to All Lab, which changes Gmail’s Reply button into a Reply All button. On the occasions you want to reply just to one person, you’ll still be able to do so by hitting the dropdown menu next to the Reply All button.
6. Google Docs Previews Even if you aren’t the biggest fan of Google Docs, the Google Docs previews Lab is pretty useful—it not only lets you preview shared Google Docs, but it also lets you view any Google Docs compatible format before downloading it. That means the next time someone sends you a Word document, Excel spreadsheet, or PDF and you don’t want to download it and open it up in Office, you can see what’s inside just by viewing its Google Docs preview.
7. Custom Keyboard Shortcuts A Gmail keyboard shortcut is another great feature of Gmail. There are a lot of keyboard shortcuts but if you use this great feature, you can change the keyboard shortcuts to your comfort. This feature could make the use of Gmail very easy.
8. Message Translation This is another important labs feature. If you have got an important email but it is in another language, you can use this labs feature to translate it to English or your preferred language. It use Google Translate for the translations.
9. Canned Responses If you find yourself doing a lot of repetitive typing, the Canned Responses lab will save you a lot of trouble. Just enable the lab, set up those messages you find yourself sending over and over again, and then send them with the click of a button. You can even send them automatically using filters and use them with external services (like automating Remember the Milk tasks). Note also that you can use OS-wide text expansion if you need to do this outside of Gmail—though what’s nice about Gmail’s canned responses is that you can use them no matter what computer you’re on.
10. Mouse Gestures You can use this as another shortcut for using Gmail. Use your mouse to navigate with gestures. Hold right-click and move the mouse left to go to a previous conversation, move it right to go to the next conversation, and move up to go back to the inbox view.
Although Gmail introduced multiple Gmail account login somewhere around March 2010 but at the same time very few percentage of Gmail users have tried it or using it. I really liked Google’s concept.
2. Click on your name/mail id shown at the right top corner. Then click on Account settings. Here is the link for those who didn’t get my point.
5. Now you can click at “on” (On – Use multiple Google Accounts in the same web browser.)radio button and save it.
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