I had used GMail Manager, which is great if you only have GMail accounts (yes, it works for multiple accounts). But if you have Yahoo! and Hotmail accounts as well, you need a bit more than that.
And, yes! WebMail Notifier also works for Hotmail and Hotmail Live (or Windows Live Mail, whatever Microsoft decided to call it this week).
Right now I’ve got all my accounts on the status bar of my browser, in one unique envelope icon (nothing fancy or shiny). When I get new messages, the proper account appears on the tooltip of said envelope. Optionally, I can get notified with a sound or a pop-up. I’m not that desperate, so I don’t have those options on; I can’t really say how it looks or works.
Anyway, give it a shot. I’m sure you’ll find it useful, if you’re a email monster like myself. ;-)