I find Stack Overflow is a great place to measure programming statistics. If I want to look at the popularity of Scala vs Clojure vs Groovy, you can just look at the number of questions tagged of each. Could we see a line graph of how many questions a tag had over time? :D
It would be cool.
(Of course, there are tons of things to take into account. Looking at just Stack Overflow, you would think C# is way more popular than Java, but other statistics say otherwise. Still, it would be a nice feature to have.)