For just over two years now I've been using Todoist as my primary to-do list manager and personal organizer software. I pay for the upgraded Premium version at US$28.99/year. I really like it and it's helped me stay on top of all the things I want to get done in my professional life, personal life, local community and beyond.
(Before Todoist, I'd been using Taskpaper and loved the simplicity of its interface and file storage. The software hit a period of being unmaintained and I really needed something up to date, so I switched. Taskpaper has since seen new life as a project, it's worth checking it out again too.)
The Todoist website linked above already showcases many of its features so I won't bother repeating those, but here are a few of the things I especially appreciate: