Switch Between Your Gmail Accounts:
A useful feature of Gmail Manager, a great Firefox extension that notifies you when you receive new Gmail messages, is that you can easily log in to a different Gmail account without entering the username and password: after adding your Gmail accounts and Google Apps accounts, select the account by right-clicking on the Gmail Manager icon in the status bar and then click on the icon.
Another way to easily switch between your Gmail accounts is the Google Account Multi-Login Greasemonkey script, but it stores the passwords in a non-secure way.
Hopefully, Google will add a feature that lets you link a main Google account with your other accounts and log in once to access all your Google accounts, like you can do in Windows Live Hotmail:
If you want to minimize the number of times you access your secondary Gmail accounts, forward all the messages to your main account and enable it to send messages using custom From: addresses.