Workflowy's tagline: organize your brain. I couldn't agree more.
Essentially they've made it dead-easy to make lists and make continuous changes to those lists. Some of their subtle features include search, keyboard shortcuts, and sharing lists with others.
I first heard about this tool on an interview with Ev Williams, founder of Twitter.
He says, "I love WorkFlowy. It fits the shape of my brain better than any other list/note-taking app I've ever used." That phrase struck a chord with me and I've been in love since.
Here's a screenshot of my to-do list: