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         If you have many Gmail accounts , then its hectic to check mails by opening all accounts . Thus logging in and again Logging Out costs much more time and opening on browsers like IE , Safari or Chrome costs efficiency of PC consuming almost whole of CPU memory , sometimes even making it Hang for a long time
A simple solution : Automatically Forward All Mails From One Gmail Account to Another Gmail
Here's how to do it by merging all gmail accounts with one Main gmail account :
Login to your Main Gmail account in which you are mostly online and receive important mails and would like to recieve mails from all accounts to this one
Go on Settings located at the top right corner of Gmail
Select Accounts
You will see :

When I receive a message sent to one of my addresses:

• Reply from the same address the message was sent to.
• Always reply from my default address (currently YourMainID@gmail.com)

(Note: You can change the address at the time of your reply. Learn more)

and below that

Add another email address   (your name and email address will be shown on mail you send)
Before you can send mail as this address, we need to verify that you own this email address.
To perform the verification click "Save Changes".
We will then send an email to the address with instructions on how to verify your address.
Name:
Email Address:
Reply-to address (optional) :

Now on second window enter the email address of your other gmail accounts which you want to recieve mails on this default main account in the Email Address box and click Save Changes
Automatically Forward All Mails From One Gmail Account to Another Gmail
You will see another window, where you have to click on Send Verification button or what you can do is just click on the email regarding verification link present at the bottom to verify that email address

Similarly add all your other email address using the same method. Verify and confirm all the email address
Forward Mails from All Gmail Address to the Primary Address

OR

Go to Gmail which is not your Main account , Click on Settings and then select Forwarding and POP/IMAP

Now you will see another box
Select Forward a copy of incoming mail to and type your Main email address
There are more three options , better go for Keep Gmails Copy in the inbox
Click on Save Changes

Now email will be forwarded automatically From One Gmail Account to your Main account

Similarly , you can check How to open Multiple gtalks , Yahoo messengers as well as MSN  for chatting with multiple accounts