I really like GMail Notifier Pro. It's standalone so you don't need your browser running, and also has a portable version. And it isn't just limited to GMail.
Unlike others I've tried, when you use it to compose/send, then the message appears in your Gmail Sent Mail. It also works well as a default mail handler in W7, as well as linking to your Google Calendar with pop-up reminders.
The free version only lets you monitor two email addresses but the license is only 13 US dollars, and it's easy enough to monitor more accounts anyway, if you just have all your other email addresses forward their received mails straight to your Gmail.
The only issue I've had, is that it doesn't seem to work with Yahoo, but that's only because YMail doesn't allow IMAP with their free account. Or something like that... I didn't spend much time on it to be honest so there might even be a work-around. Correct me if I'm wrong there anybody.
It will add a few seconds to your boot time, but I like it enough not to mind that.
Also it's very very easy to set-up and configure ... no messing around with server addresses, ports, or anything ... just give it the email address and the password and that's it.
Lastly the developer seems very helpful... I asked for a (completely pointless) Sent Mail Confirmation pop-up ... "Your Mail Has Been Sent" kind of thing, and that was added to a new version within a couple of weeks I think.
And, I agree with ppass... WebMail Notifier is also great if you have Firefox running all the time.