Many have been eagerly waiting for the Leopard preview since it's announcement in 2005.  It is already known that Boot Camp will be a part of Leopard but expectations are that Leopard will usher in at least some of the following:

• Animated icons
• Revamped Finder with Spotlight integration
• VOIP Telephony
• Inkwell - Handwriting Recognition software
• Voice Recognition and Optical Character Recognition improvements
• Prefetching
• iChat tabs
• Improved iPod integration
• Home integration

Information on the Apple Developer Connection web site indicates that "Session Tracks" will focus the following five areas:

• Application Technologies
• Graphics and Media
• Development Tools
• OS Foundations
• Information Technologies
  

Rumors indicate that Leopard may also usher in virtualization software that will allow Mac users create and run virtual machines within OS X.  In addition, expectations are that Intel powered PowerMacs will be introducted albeit with a new name.