Workflowy is a tool I'm using for a long time now. Here and there I got distracted from using it but for all my personal projects workflowy finally came into play to keep track of my stuff.
Workflowy, basically, is a never ending list of tasks that you can append and manage. With tabbing, you can declare the parent of your task. Clicking on a tasks bullet point shows you the selected task as the top most of your current view. Then you can add other tasks to it. This way you can manage your tasks many layers deep which keeps them clear for you to track.
They have an app to quickly write down your thoughts from the phone, which really helps me when I think about a project I'm doing.
You're able to add 250 tasks every month in the free edition which is enough for me. You also can start tricking and rewrite already ticked tasks, which doesn't feel like a good workflow, but it works. If I start using it excessively I would rather pay their premium model than rewriting tasks all the time.
Take a look at workflowys blog to get a peek behind the scenes.