There are a million to-do list applications out there, both rich client and web-based. I never found one that did everything I wanted it to, which led me to consider writing my own. But then I discovered Todoist and now life is so very good.
Todoist is a web app that gives you a standard list of projects and tasks. Projects can be color-coded, and tasks can have a priority, labels, notes, a due date, etc. Both can be reordered manually or in the case of tasks by priority. You can further query by label or due date, and move tasks easily between projects. The killer feature for me was you can create subtasks below a main task, which show as indented and can be marked as completed individually while keeping the main task active.
They decided to use very a Gmail-like approach to the user interface. Clicking on one of the gear icons next to each project and task activates a context menu similar to the ones in Gmail. It’s fairly minimalistic in the overall layout of the page, making it feel very snappy. Plus it has a bunch of keyboard shortcuts and there is not only an iOS app but a Firefox extension for it as well.
They offer a base set of features for free, or you can get the premium version for $3 a month or $24 for a year, which gives you the advanced query capabilities and some other stuff. It was well worth it.