Basecamp is an excellent online software package that makes project management and collaboration easy.
Created by 37signals, the entire Basecamp system is accessed and managed online, so none of the software or data is actually stored on your computer. You can share the project management with your fellow freelancers or an entire company.
Basecamp helps you manage multiple projects at a time with to-do lists, file sharing, chatting, messages, calendars and time tracking. 37signals offers a trimmed-down free version, which is great for starting out, as well as monthly plans.