There are lots of all-in-one solutions (Co-Schedule seems to be one I see popping up in my Facebook feed a lot), but we cobble together our own. Google Docs for editing, WordPress editorial calendar for planning, Flow for assigning and completing tasks.

We manage a number of high-volume blogs, and we use the same system across the board to keep it simple for all of us. This post goes into detail on how we do it -- happy to answer any questions: http://business.tutsplus.com/articles/a-proven-step-by-step-process-for-managing-multi-author-blogs--fsw-44944?_ga=1.128639300.974525869.1404268441