Most semesters, I teach a class in Software Engineering. The centerpiece of the course is a group project using Scrum, and the students absolutely require a good web tool to organize their work.
I have a number of requirements for a Scrum tool:
It has to be free;
It has to be easy to use;
It has to implement some basics of Scrum:
separate product and sprint backlogs,
It has to be easy for me as an instructor to follow my students’ progress.
Moreover, I want whatever tool they use to implement the basics of Scrum and not much else; teams should be free to work how they like within the bounds of Scrum without the tool imposing its own viewpoint.
For a while I used one tool until it was no longer free; then I used another, but it didn’t really do what I wanted, etc.
Finally, I gave up and wrote my own, and the result is Scrumboard.
Scrumboard makes a few choices of how you might work within Scrum, such as Epics (which group Product Backlog Items), Acceptance Criteria (which define how to know when a story is implemented), and Tasks (which define how work is broken down within a sprint). All of these are optional, but I think they’re pretty lightweight solutions to common needs when working in Scrum.