Bitrix24 is a social intranet, a unified work space for your company. The service enhances internal information exchange and communications and places a complete set of business tools into a single, intuitive interface.
[Top Ten] One of the best collaborative project management solutions
Bitrix24 is one of the best collaborative PM out there and being free certainly does not hurt. It looks like creators really studied competitors and created a solution that overcomes shortcomings of Basecamp, Asana or Zoho Projects. Mobile app is excellent but doesn't have Gantt, unlike the web version. That's something to work on.
Tips for using the product
Pros: free plan, lots of features, good for collaboration and task/project management