WinZip is the de facto standard for working with zipped files and a program I have used for longer than I can even remember. I just received a notice from the WinZip folks about the introduction of the new WinZip Companion for Outlook add-in.
The Companion can be configured to automatically zip your attachments, to ask whether or not you want them zipped, or to let you zip and attach files manually with just a few mouse clicks. Toolbar and menu items allow you to control the Companion’s functionality on a
WinZip for Outlook looks like another useful utility from these folks and will retail for $19. There is a 45 day trial available at the WinZip web site.