Very interesting report with data about independent software developers including mobile devs, webdevs, and desktop devs. (I guess that fits most of us here).
One caveat: to download the report you’ll need to register on the site.
/edit: Here’s an article summarising the report if you don’t want to register: itworld.com/article/2957215/development/how-to-make-money-as-an-independent-developer.html
(Eh, sorry can’t make the last link clickable - the forum tells me I’m not allowed to post links because I’m new …)