The newest version of Macromedia Flash. It supports many more interactive abilities, like:

• More scripting power - now Actionscript looks and behaves more like javascript
• The Flash player is supposed to play movies faster than the Flash4 player
• You can hide the cursor
• You can draw Bezier curves in the program, instead of having to import them from Illustrator or Freehand

Many, many more things, but I can't think of them right now.... just go get it.