Does anyone know open source CAD software (like edit, update cad files and attach external information, create topology etc.)
Researching on internet also (Going through Kosmo Desktop, DraftSight etc.)
LibreCAD is one open source project that I found promising. It's GUI is very similar to AutoCAD. This makes it quite easy for AutoCAD users. At present it does not support .dwg format. But you can work on dxf files.
If you're looking for free as in I don't want to pay money for a CAD program, I recommend you look into Draftsight
edit: Just in case, you wanted to learn the software or you're coming from AutoCAD, I've developed this tutorial / cheat sheet:
http://220.127.116.11/Autocad_tutorial_v1.pdf a few years ago. Unfortunately, since it's a numeric IP address, Stackexchange, doesn't allow it to be a link
DoubleCad XT is free but not open source. Its functionality is compared to AutoCAD LT. Only runs on Windows. There is no scripting component.
Librecad and qcad are 2 related projects that provide een open source 2D CAD-solution. FreeCAD is a 3D CAD suite, based on python en OpenCascade. Not very usefull for 2d drafting in my opinion, but very cool with all the python scripting stuff. None of these suites support dwg because licensing reasons.
Draftsight is one the most complete Free ( as beer but not as in Freedom ) 2D CAD Applications. It does supports DWG. It works on windows, linux and mac and is made dassault, the maker of the 3d modelling suite solidworks.
teigha_viewer can open and convert dwg to dxf: usefull in combination with Librecad or QGIS.
BricsCAD is an alternative. Although its not free, it mimics AutoCAD almost verbatim and its less than 500 bucks for the Classic version and only 825 for the Platinum. I have been usung it in practice for almost two years with no hiccups.