Michael Feathers wrote a blog about Open-Closed Principle, where he described simple technique that measures the closure of the code. I created a Groovy script which implements this algorithm for Git repositories. If you run it from the root of your Git project, it produces a CSV file with the statistics of how many times the files have been modified.
As an example, here is the top 10 files from rabbitmq-server repository
845 src/rabbit_amqqueue_process.erl 711 src/rabbit_channel.erl 650 src/rabbit_tests.erl 588 src/rabbit_variable_queue.erl 457 src/rabbit_amqqueue.erl 448 src/rabbit_mnesia.erl 405 src/rabbit.erl 395 src/rabbit_reader.erl 360 src/rabbit_msg_store.erl 356 src/rabbit_exchange.erl