Wolfram CDF Player (was Mathematica Player) is an innovative new take on viewer applications. Rather than just a reader, like Acrobat Reader, or a thin runtime, like Flash Player, Mathematica Player has the full Mathematica engine embedded right there-ready to make documents come alive or to power applets. (Of course, don't forget it can read notebooks too!)
Because Player-interactive documents are so easy to make using Mathematica 6 technology, thousands of these are already freely available at The Wolfram Demonstrations Project-with more supplied each day by researchers, educators, students, and professionals.
CDF Player includes all the great functionality you've come to expect from Mathematica Player-including full support for .nb and .nbp files-plus new interactivity and deployment options.