Asana is a hybrid task and project manager. We covered the service when it launched back in 2011, and since then it's updated several times, spawned iOS and Android apps, and boosted its collaboration features for both individuals and organizations. Adding multiple projects is simple, and you can keep track of them from the left sidebar. You can structure your individual project goals and milestones as a simple checklist from start to finish, order them by date or when they need to be done, or make them dependencies so one thing can't be complete until its sub-tasks are finished. You can add more detail to any task or item, like notes, links, tags, and comments, and if you're working with others, you can see changes they've made as well. Upload attachments, set due dates—it's all there. Plus, Asana packs tons of keyboard shortcuts that make using it fast.
Asana is free for most people (you only need to pay once you get up to 15 or more people working on the same projects), and it's just as good a corporate project or task manager as it is a to-do manager for your own pet projects or ideas. Best of all, they don't pare down features in the free tier—all of the functional features are the same, with paid users only getting things like priority support and "guest" users. Companies like Dropbox, Pinterest, and Uber use Asana to organize their projects, and—full disclosure—so do I. It's worth checking out if you don't have an account.