I am an avid user of Redmine project management system on all of my freelance websites. I can't imagine how I managed my projects before. IMHO it is single best collaboration tool to use with my clients. There is sum functionality however that I just didn't want my clients to be able to manipulate. For example, when a client creates a new issue, I don't want them to have the ability to assign the ticket to a specific user. More than that, there were other little bits of html I wish I could just hit for clients. So my solution was to hack redmine!!!