Ask HN: Recommendation of open source web project management tool

I'm currently looking for an open-source web project management tool that is good looking ( 2013 ).

I'm searching something more similar with tree.io than with a To Do list.

This will be used by a team of volunteers supporting a team of trainers for education purposes.

I really appreciate some recommendations of software with usable UI, the kind easy to understand be easy to be used by non-technical people and in the same time appealing.

List of what I already tried:

Tree.io (almost complete - lack the per project permissions/team members)

Clockingit ( too focues on customer/time spent than on project management and tasks )

Track and RedMine ( flexible, old UI, unappealing and focused on technical users )

Getontracks ( lack the ability to assign tasks to various users )