Horde

I have tried many PHP based groupware solutions like eGroupware, PHP Groupware and some others taht i don't remember, but this one is the best from my point of view. It has a clean UI on browsers and also on mobile with state of the art features like notifications, ajax, ... . But the best is that you can install only what you want or need through the PEAR installer. Also updating is quit easy through a simple 'pear upgrade-all' so taht a fixed bug doesn't need a complete reinstall or update of the entire groupware. if you need a calendar, a task management or a simple notepad simply install it and it integrates seamless into existing modules. Horde also supports ActiveSync which is really nice if you have an Android or Iphone, so you stay in sync with emails, calendar and adresses. When you find a bug simply fill a report and you will get a comment or answer from the developers in a short time. Fixing bugs is also yuit easy with git so you can push the fix and it will be reviewed also in a short time and integrated.
The developers do a great job and if you still need a special feature there are also campaigns which you can sponsor.