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.