Whether you are working on a project with a team or alone, there are always multiple variables to keep track of. There are many tools online to help you stay on top of things.
For more complex projects: The collaborative project management tools in this list include multiple features for team and task management, such as managing deadlines, assigning tasks, time tracking, reports, alerts and notifications, wikis, uploading files and versioning, charts and graphs, chat with team members, and more.
Planzone (includes a wysiwyg wiki that is very useful)
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For less complex projects: These collaborative project management tools have some of the above features and are ideal for smaller projects and smaller teams or individuals.
But sometimes all you need is a good to-do list. Below are some of the best.
Doris (my favorite)