ThinkComposer is described as 'Create deep visual documents, adaptable and multi-level diagrams, flowcharts, concept maps, mind maps and models' and is an app in the Office & Productivity category. There are more than 100 alternatives to ThinkComposer for a variety of platforms, including Windows, Mac, Online / Web-based, iPhone and iPad. The best alternative is Joplin, which is both free and Open Source. Other great apps like ThinkComposer are Standard Notes (Freemium, Open Source), Google Keep (Free), Microsoft OneNote (Freemium) and Evernote (Freemium).
Joplin is a free, open source note taking and to-do application, which can handle a large number of notes organised into notebooks. The notes are searchable, can be copied, tagged and modified either from the applications directly or from your own text editor.
Draw lets you produce anything from a quick sketch to a complex plan, and gives you the means to communicate with graphics and diagrams. With a maximum page size of 300cm by 300cm, Draw is a an excellent package for producing technical drawings, general posters, and much else.