If you work in tons of different Google Accounts every day, the new Chrome beta is worth a look.
Instructions geared to AdWords agencies, courtesy of J Prentice on LinkedIn:
1. Download Google Chrome Beta – http://www.google.com/landing/chrome/beta/
2. Click the “Customize and Control Google Chrome” wrench on the right side.
3. Sign in with your first MCC email address.
4. Back to the wrench > options > personal stuff > add new user.
5. Sign in with your second MCC email address
6. Click the icon in the top left of your browser then “edit”.
7. Choose your icon and rename the account.
8. Rinse and repeat for as many MCCs as you have.