I use more than one Gmail account -- a personal one, and one for each of my web sites. To save time logging in and out of each of my account, I use the Firefox browser add-on called Gmail Manager.

Once installed, you just add each of your Gmail account, together with the password of course, in the Preferences window. The Gmail Manager now appears in the right portion of the Status Bar. To log into an account, just right click the Gmail Manager and left click on the the account you want to log into.

Once logged into an account, you can mouse over the Gmail Manager to view the latest messages. You can also right click and then left click on the Show new mail snippets menu item to view excerpts from the latest messages. Left clicking on the Gmail Manager meanwhile will log you into your Gmail account itself.