The application I use is
Mailbox Verifier, handyman. The reason I like it is that it has a very small footprint and just sits in my System Tray checking for email while hardly using any resources.
However, while it will play a sound - and show an animation/change colour - for new emails, it won't do so differently for different accounts, so it may not be what you're looking for.
My ideal is something as near as possible to the classic Netscape nsnotify: I don't want bells and whistles, just a small app that does the job. To my mind, the big problem with most email checking utilities is that people keep adding clever functions to them until they're almost email clients in their own right. You might just as well leave your email client running minimised...