Basecamp
Basecamp is an online project-management and collaboration tool developed by 37signals.
Basecamp users can create to-do lists, chart milestones, track times, share files with co-workers and clients, and utilize the message boards to comment on progress or thoughts related to the project.
Basecamp offers a free one-project trial and paid plans for $49/mo for 35 projects, $99/mo for 100 projects and $149/mo for unlimited projects.