For the past 12 years, KDNuggets has conducted an annual poll asking "What analytics/data mining software you used in the past 12 months for a real project (not just evaluation)". In this year's poll, R was the top-ranked data mining solution, selected by 30.7% of poll respondents. Microsoft Excel was second, at 29.8%. Rapidminer, which took the #1 spot over R in 2011 and 2010, ranked third. And as Bob Muenchen notes, four of the top five ranked data mining solutions in this year's poll are open-source. R was also ranked in this poll as the most popular language for implementing data mining application, beating out SQL and Java.
See the link below for the complete list of tools ranked in the KDNuggets poll.