I have been using Mailfiler (www.mailfiler.com) for about a year now. Excellent for folks who receive 50+ e-mails per day that need to be broken down according to project, client, etc. and/or kept for reference (newsletters, etc.). Also, it sorts your inbox into multiple groups automatically based on sender (i.e. personal vs. clients, vs. intercompany, etc.) Allows messages to be autofiled (even to the hard drive) and has an alert/reminder/follow-up feature that blows away the standard outlook alerts. Also allows you to sift through an e-mail thread in the same way you would in a forum setting. Works with pst based or exchange-based outlook configurations.
I've found it to be very useful; however, if you store a lot of e-mail it can slow down your Outlook performance a bit.