I'd just like to say up front: these guys are getting all my donations for the week. Put simply, Act-On is a plugin that allows you to assign keyboard shortcuts to rules in Mail. Anything that's possible with a rule is now just a keystroke away. Read on for an example of how it works and how much power Act-On can give you over your Mail messages.
There's a simple naming process to follow for rules, but once you create one you simply must use the (`) key (above Tab) to invoke Act-On, then use any of your shortcut keys to trigger a rule. To give you an example to get the gears turning: I've now set up 4 separate rules, each to color the background of a message in Mail's list a different color. Coloring messages quickly and easily like this helps out tremendously if I need to find a specific message in a list or search result, but you can do so much more. Any rule you can conceive - say one that colors a message, moves it to a folder and runs an applescript - can be called with a simple Act-On shortcut. Powerful, and best of all: handy. Just like MailTags, Act-On requires Panther or Tiger, and a $20 donation is recommended.