Elements of Statistical Learning (Online) 

 In case you had not already heard, Trevor Hastie, Robert Tibshirani, and Jerome Friedman have put a PDF copy of the second edition of their excellent text  Elements of Statistical Learning  on the book's website. I am sure many of you already own it, but a searchable version for the laptop is incredibly useful. The second edition has a lot of new content, including completely new chapters on Random Forests, Ensemble Learning, Undirected Graphical Models, and High-Dimensional Problems.  

 While a copy on your computer is very handy, a desk copy of this book is essential if you are interested in machine learning or data mining. The book is also a sight to behold. You can buy a copy at  Amazon  or  Springer .