Software development organizations are facing a paradigm shift towards Distributed Software Development. This shift introduces situations from which organizations may benefit (e.g. highly skilled human resources, development groups closer to client location, etc.); but also introduces challenges to which organizations have to adapt (e.g. coordination difficulties, inadequate knowledge management and communication, and lack of inter-virtual-team trust relationships). In this work, we particularly study the lack of timely adequate opportunities for informal interaction, which has been identified as an underpinning foundation to overcome coordination, communication and trust limitations. To achieve this, we introduce and define the concept of Collaborative Working Spheres (CWS), through which developers can obtain information related to the personal activities of their distributed colleagues. CWS allow identifying opportunities for collaboration in suitable moments both for the one making contact and the one being contacted. We notice that other examples of technologies, including the telephone and instant messaging are used by developers for starting collaboration; however, they do not provide enough information from the personal activity of the person being contacted. We argue that with CWS, software developers will be able to become aware about the status and progress their partners have achieved in some activity, and use this information to inform their starting collaboration. We illustrate this concept with the design of a CWS-based messenger tool that supports Collaborative Working Spheres for Distributed Software Developers. The results of an initial evaluation provide encouraging evidence on the perceived usefulness and ease of use of the proposed CWS-based messenger tool.