I'm a recent pc convert and always used poppeeper and mailwasher. I've just spent HOURS trying to find an equivalent for the mac, I could find plenty that just told me I had mail or how many but none that would show a small preview of th eheader etc a la mailwasher. But finally I think I found one.
http://www.popmonitor.com/
Even the unregistered version is perfect for what I need and I hope this helps!