The use of open source software has been increased rapidly due to its various benefits. These days many open source software exist that provide different capabilities for users to handle with spatial data. The selection of an open source software with needed capabilities for special project and lack of any responsibility about these software from corporations, forces the members to use their experiences. The forums give these opportunities to users to transfer their experiences in global level through Internet environment. In this paper, three top open source software, QGIS, GRASS and gvSIG have been investigated using questions and responds in the important GIS forums. Also these software via forums and forums capabilities have been compared. The results show that QGIS is used more than the others. Also stack exchange is the appropriate forum for ordinary GIS users and OSgeo is the suitable forum for expert GIS users.