Depending on what you mean by "non-technical", I'm voting technical writing skills. Mark Freedman is right to mention (interpersonal) communication skills, but technical writing skills are also important for a developer.

If nothing else, every comment you add to your code is a snippet of technical writing. Also, no matter how good your spoken communication skills, a written explanation of what you understand the requirements be that others can understand is very important - without it, even if the initial spoken communication was perfect, yours and your bosses/customers perceptions of what you're supposed to be doing will inevitably drift in different directions over time.