Outline-based hierarchical task manager with tags/due dates/markdown; PRO version (~$39/year) has notifications, assignees, repeating due dates, etc.
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Pro Tasks can be managed with custom tags
Pro Great search
Search filters help you search for tasks by tag, due date, assignee, etc.
Pro Converts incoming emails to todos - this is a way to enter tasks from anywhere.
Pro Convenient search by text or tag filtering
Pro vim-like commands
Pro Very flexible
To-do lists often need to be grouped and regrouped for clarity. The outline-based nature of Checkvist makes it easy to move things across categories, to increase and decrease priority, etc.
Pro Smart parsing of items
Parses markdown, html links, hashtags to become formatted text, clickable links, and tags, respectively.
Pro Supports comments on tasks
Any item can have a free-text comment, which can be shown or hidden as desired.
Pro Generous set of free features
Checkvist offers many free features if you are using this individually.
Con Limited capability in the android app
Checkvist is primarily a web interface.