Data Structure Visualizations
I dream of a world where this is built in everywhere that I write expressions.  It's so hard, and we desperately need it.  Bret Victor has called a lot of attention to this lately, and rightly so.It's easy to forget how much we have to remember inside our heads just to work with code.  And why?  Much of this work could and should be done by the machine, which would free us to focus on what we want.  But the need for context, different execution environments, etc, all make this difficult to tackle generally.More and more lately, I find that I'm interested in this problem more than the code itself.