Favorite Windows email tricks and plugins

I’m working on an article about email tricks for one of your finer magazines, and – as you might imagine – when it comes to the inevitable Windows stuff, I’m a bit light in the useful tips department. So, I turn to you Redmond-using smarties for help.

Do you have a favorite application, plugin, trick, or hack for bending Windows email to your will? Double-credit for Outlook add-ons that garden-variety users can install without fancy root-style access. Whence comes your magic Windows fu?