It rather depends on what you are going to do with the 3D you may want to use something like Blender then you can export to STL format for 3D printers etc. What is your application ?
Tinkercad is the online version of 123D (more or less.) Check it out at https://www.tinkercad.com/. It has pretty much the same look and feel but if you are used to 123D you may find it a bit disappointing.
