EmailTray monitors all of your POP, IMAP, and Web based mail, and pops a notification in the System Tray when new messages arrive. That's a cool trick all by itself, but it gets better. The program automatically sorts your mail into categories of urgency: High, medium, and low, as well as suspected spam. It figures out the priority levels automatically based on the "interconnectedness" of you and the recipient (and you can adjust it as well).
Here's where it gets interesting. EmailTray is easily configured to only alert you when mail of a certain priority arrives. So if you want to know when a high priority message arrives, but not medium or low pri messages, that's easy to do (and is, in fact, the default).
That part is awesome. From there, the program goes into meh territory. It has its own inbox interface, which aggregates all your mail into the aforementioned high, medium, and low categories. You certainly don't have to use this window, and in fact I studiously ignore it, preferring to open Outlook directly. But I can't help but think that recreating a second mail interface is a poor use of developer resources.
Whether you like the standalone EmailTray inbox or not, though, the notification system and automatic prioritization is superb, and well worth installing (especially since it's free).