This is a good question. I know a lot of people, myself included, get stuck in a rut where we are just churning out code like one would churn out burgers at a McDonald's. Coding becomes too mechanical—we understand how to get things done, but often times we forget why these things get done behind the scenes.
In my world it's been C++/C/ObjectiveC that have taught me the most, even though I write C# every day at work.
For the most part, learning C++ has helped me learn about memory management and how the various objects are stored in memory, etc.—the actual science of programming. What really opened my eyes was the Programming Paradigms class offered at Standford that you can get off of iTunes and I think YouTube.
