If you like to plan and brainstorm by creating mind maps to link together ideas, MindMup is a great browser-based program that helps you get organised with lots of features.
The app is a lot like previously-mentioned Coggle, but it offers a few more features that power users will love. The biggest difference is the ability to augment your nodes with various right-click options. For example, you can attach long notes and images to your nodes and click through to see them later so you don't have to clutter your map with too much top-level information. I also really enjoy the ability to drag and drop nodes on top of each other to reorder them in the hierarchy. These little touches make the app feel more native, and less like a typical web app.
One much-appreciated feature is the ability to save your maps, either to Google Drive or MindMup's own servers, to edit at a later date. You can also export your maps in various formats, including PDF and HTML, offering you a ton of flexibility when it comes time to act on your data. Granted, not everyone likes using mind maps, but if you do, take MindMup for a spin.