In a strange deviation from the norm, Google has taken a Google Apps-only feature and added it to regular Gmail. Email delegation allows you to grant other Gmail users to access your account (without signing out of theirs).
While the original feature was useful for folks with personal assistants, it can really apply to any situation in which you'd like someone else to access your account, but not change any of the settings. Delegates can view your inbox and respond to messages on your behalf, but they can't chat, change your password, or modify other certain settings. They can also view both your account and theirs at the same time, and either view them simultaneously or switch between them quickly from the top of the Gmail window. To set it up, just go to Settings > Accounts and add another account under "Grant Access to Your Account". Check out the video above to see it in action.
Email Delegation: Granting Access to Your Gmail Account [Official Gmail Blog]