By far, the most visited post of 2011 was the "What Do Data Miners Need to Learn" post from June.
The top five visited posts that were first posted in 2011 are (with actual ranks for all posts):
1. What Do Data Miners Need to Learn
2. Statistical Rules of Thumb, Part III
3. Statistical Rules of Thumb, Part II
4. Number of Hidden Layer Neurons to Use
5. Statistics: The Need for Integration
The top six viewed posts in 2011 originally created prior to 2011 were:
1. Why Normalization Matters with K-Means (2009)
2. Free and Inexpensive Data Mining Software (2006)
3. Data Mining Data Sets (2008)
4. Can you Learn Data Mining in Undergraduate or Graduate School (2009)
5. Quotes from Moneyball (2007)
6. Business Analytics vs. Business Intelligence (2009)
The "Free Data Mining Tools" post is understandably relatively popular, even after 5 years. The Moneyball quotes has a particularly high bounce rate. I'm most surprised that the K-Means normalization post has remained popular for so long.