Visibility analaysis plugin is free – but there is a lot of work investend into it. Help the future developement of the software.
Goal for the next major version: integration into processing toolbox
Many QGIS users have experessed a wish to integrate visibilty analysis in custom scripts. That would be, indeed, a very useful thing. For instance, one would be able to combine viewshed with morphometry and other terrain analyses, or do some complex statistics in a batch. However, this would imply a major overhaul of the code – your help will speed up the work.