Without a doubt one of the greatest changes in the last decade has been the migration of e-mail from the realm of a small portion of the population to the pervasive force it is in our life today. Users of Apple's Mail application have a great tool available to help turn the mass flood of email into a manageable stream of information. Mail Act-On is a plugin that makes it possible to do any number of things to an email box with the press of a key (well two keys really).
In addition to the obvious things like highlighting email and doing things when email arrives in the inbox, Mail Act-On makes it easy to perform rule actions at a later time. I generally find two pairs of rules are quite useful. One pair files email in the archive - depending on which account it was sent to, and the other files them in an action folder, again depending upon which account they are in. This makes it easy to have a single inbox and to whip through the email while deciding whether something is to be replied to immediately and then archived, simply archived, or put into an action file for later follow-up.
Mail Act-On is a great tool for having an email box that is empty at the end of the day and being more productive while doing it.