1. Getting Things Done: The flow chart he provides is the key feature of the book. Most of the other advice is mediocre, in my opinion. https://lifedev.net/wp-content/uploads/2007/02/gtd-workflow....
2. org-mode: It is incredibly powerful, honestly, the most powerful productivity / todo app. However, I found that this power slowed down my process, and I find that kills my productivity. If you love powerful, complex tools, org-mode is for you.
3. Todoist: it is an adequate list app; if thats all you need, it works well. Also, it has an open api, but I never used it.
4. Todo.txt / Markor: this is my current setup. Markor is a really nice android app; https://github.com/gsantner/markor . I generally like the philosophy behind todo.txt. It forces you to simplify your todos. My main problem is that notifications aren't a part of the programs at all.
I hope this is useful to someone.