Mapping and georeferencing software

I would like to talk about various options for mapping and georeferencing. We have dabbled with Google maps and Hypercities and are exploring other options. I would like to have a better understanding of what open-source software will work well enough and when the more advanced features of commercial software are needed. I would like to learn what software people have been using for what projects and the level of expertise required for the software.  I would also like to talk about open-data in mapping projects and also having data in open formats to archive (even if the data is not openly shared).