Graphics & photos - including jpeg and gif images
Most banner ads - including animated gif images and Macromedia Flash content
All browser-based emails
Emails and attachments
Outgoing SMTP email - especially acceleration of large messages with pictures or attachments including Microsoft Word, Excel, and PowerPoint files
MP3 files, executables & other files - when downloaded from within your web browser (depending on the quality of your connection)
Secure pages, such as those used for online banking and credit card forms and pages accessed over VPN. Why, this is actually a good thing.
Streaming media, and audio and video files
Files downloaded outside your browser using ftp.