I just came across a splendid tool that is free and easy to learn. Bubbl.us allows students and teachers to brainstorm, establish hierarchal webs, and much more. It's simplified format makes it great for students to use, yet the text buttons can be customized to fit a variety of tasks. Plus, it's easy to embed in your website or other documents. Sounds like an ad, doesn't it? Really, I'm just enthused about how I'll use it in my classroom. I'm thinking that plot sequencing and web diagrams are perfect applications. Let me know what you're doing with it!
Here's the link to Bubbl.us: https://bubbl.us/