Tried so many apps, never really got settled, I think this one will be the one for good. I'm loving it so far, it's clean, easy to use, and yet very powerful. The client handling aspect is just too epic.
Clean and functional tool, that is pretty mature, with a great team behind it. :)
AC is on the project management market for a long time, and their team has tremendous experience in building and improving project management tool.
I purchased the self-hosted version and have used it for the last year and a half.
I've been using ActiveCollab for quite some time now, and it really helps me organize my work and projects. I love the fact that I don't need almost any other tool to run the daily stuff, since everything can be done through AC, from the beginning to the end of the project.
In short, ActiveCollab incorporates project management, time tracking, and invoicing into a comprehensive Project Management Software.