I've been working on the outline for my first juvenile mystery, and I decided to give the web-based tool Trello a try. It's not actually designed as an outlining tool; it's a project management system. But I saw a tweet from someone who mentioned using it for outlining novels, and I thought I'd give it a try.
I wouldn't say it's the perfect solution. It's a virtual card-based system and at times the visible portion of the cards seems a bit small for my purposes, but then that's one of the problems with using a system that's not actually made for what you're doing. With a little care in phrasing the first line of any card, I can work around that problem, and Trello has other advantages.
I use a column of cards for the plot points, one to keep track of the things I need to research (yes, research is important in juvenile fiction, too), one to keep track of characters, one to keep my plots and subplots in order, one to keep track of the locations in my fictional town, and another for notes. The last column is labelled "done" which is where I keep research I've finished, plot points I've written prose for, and notes that have made it into the story.
It's a lot to keep track of, and Trello manages to keep it all on one screen on my computer. Maybe when I've done one entire book using it, I might decided I need something specifically designed for the task. But so far, it's working out just fine.