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.