Now, you don't have to sign in, all the time you use Gmail. Just sign in once, and Gmail will ask you, if it has to remember the account for future references.
Once you click on 'Remember this account', even if you sign out of that account, Gmail remembers your account and gives you an option to click on the account and signs in automatically!
What more! You can even add all your other (gmail) accounts, and ask Gmail to remember them. This way, the next time you want to sign in, Gmail shows you the entire list of 'remembered' accounts, you just have to click on it and you are signed in!
You can even remove accounts, by clicking on 'Edit'.
How to enable this feature? Just sign in to Gmail, and add an account. Once the page to add a new account appears, in the footer you will see a text 'Coming soon a new sign in page: Try it now'. Click on it, and you are done!
Here is what Gmail has to say about this feature:
Add your account to this device
Tired of signing in so often? Add your account here. New apps (like a shopping website) may even let you sign up just by clicking the account or entering its password.
Sign-in made easy
1) Once you add an account to this device, you won't have to type your email address whenever a page asks you to sign in. Just select the right account from a list.
2) Some pages will ask for your password, but others will sign you in automatically after you click the account name.
More help available on Google Support: http://support.google.com/accounts/bin/answer.py?hl=en&topic=14129&answer=1691641