Web/iOS/Android: Kona is a free to-do manager that allows you to create multiple workspaces for multiple projects. It features a dashboard view that lets you focus on one task at a time as well as see your overall progress, making it the ideal solution for tracking team-based projects.
Fans of the defunct Wunderkit may recognise Kona's layout and approach to project management. Both tools allow you to isolate and categorise tasks, and they let you invite others to work on a project without seeing the others. You can customise the look and feel of the workspace with different colours and icons to make it feel more personal.
Kona can also add meetings, milestones and other events to the sidebar so that you don't have to check a separate calendar. Plus, it offers free mobile apps for iOS and for Android so you can keep up with your projects on the go.