At the risk of sounding greedy, I have five Gmail accounts. Hey, Google makes the rules, not me. Gmail accounts are free, and there's no limit on how many a single person can have. However, checking all those accounts gets to be pain, what with all the signing in and signing out. That's why I rely on one of my all-time favorite Firefox extensions: Gmail Manager.
True to its name, the add-on lets you manage multiple Gmail accounts from within the comfy confines of your browser. After installing it and configuring your accounts in the Preferences, you'll see a Gmail Manager status bar in the bottom-right corner of the browser window. Mouse over it for a pop-up listing your newest messages. Click it to open Gmail in a new tab. Or right-click it to select a different account.
Gmail Manager has loads of options you can tweak, like new-mail notifications, a numeric unread-mail count, and a pop-up "snippet" box. In short, it does everything you could want short of actually reading your mail for you. This is a killer extension and a must-have for anyone who uses multiple Gmail accounts.