Can one of your letters of recommendation come from a professor in a department unrelated to your field of study? I took a research writing course at my university (which is required of most students) and did extremely well in the course, winning a writing award for the work I did in that class and having it published in the school's textbook for their introductory English courses. I went on to be a TA for that professor for two semesters afterward and did extremely well, and I think his recommendation would be nothing but glowing.

However, I am an Electrical Engineering major applying for an MS in Computer Science, and while his recommendation shows my ability to write and teach, the work I did was entirely non-technical. Would it still be worthwhile to ask for a recommendation from him, or should I focus on getting more letters from my EE/CS professors instead? I currently have one excellent letter from my employer (a small company that I worked for a little over a year during school, was able to lead development on a few projects and teach some people, mostly industry), but sadly don't have a good enough relationship with enough of my other professors beyond "got an A / did well on some projects".
I can't think of how an excellent letter of recommendation from a professor in an unrelated field of study could hurt you. Perhaps if it was the only (excellent) letter of recommendation, maybe then. But you mentioned you have an excellent letter from your employer and can (I assume) get at least one more good letter from someone else, so I think you are in a good position.

You are right, his letter of recommendation would not be about your technical skills related to EE, but it would mention your characteristics, work ethics, teamwork ability in a very good light. Research writing course is also not that far away from EE. It shows you are more diverse than the stereotypical programmer or engineer. You worked with this specific professor for 2 semesters, he could give you an outstanding letter of recommendation. I think his letter will give you more than any single average/solid letter from other professors.