At WordUp Minneapolis 2014 I gave a talk on using WordPress for project management.
In this session I talked about building a simple project management app with WordPress using the P2 theme and a handful of plugins. With this app you can create to do lists, check off those tasks, and mark posts as unresolved or resolved.
The base of this tool is the P2 theme, which allows you to easily make new project posts from the frontend of the site. This theme is extended with P2 Resolved Posts, P2 New Post Categories, and a custom plugin of mine called P2 Projects. I also use the Members plugin to make the site private.