Secure and Zip your Email Attachments
PKWare (the makers of WinZip) are giving away free licenses for SecureZIP. This allows you not only to ZIP up files but also to encrypt and sign them using industry-standard encryption methods. What is even better is that this integrates seamlessly into Microsoft Outlook and allows you to zip up (and/or encrypt) attachments by default. That has the potential of seriously reducing the size of the mails you send, and receive (provided you can persuade enough people to install it!).
To pick up you free license, go to http://www.securezip.com/, enter your email address* and you will be sent instructions on how to download.
* Remember that if you don't feel like giving out your real address, you can always use a service like Spam Gourmet.