If you are working on a project with a large team, then its really difficult to collaborate without using a good project management software or service.
Project management software and services makes project management very easy by handling common tasks such as project planning, remote employee tracking, document and media sharing, tracking time and deadlines, task delegation and management, reports generation, proper communication etc.
Earlier many offices used Microsoft Excel to manage projects and track deadlines. But it cannot handle most of the project management tasks listed above. The best way to manage project these days is to use an online, web based, project management software or service. There are hundreds of such services available. After checking most of them, I have concluded that the services and software listed below are best for online project management.
• Zoho Projects: 
Features include milestones, calendar, scheduler, meeting planner, document sharing, time reporting, forums. They keep adding new features so do check out what they offer now. The free project management plan lets you manage only 1 project at a time.
• Basecamp: One of the most popular and best service that is used by top companies. 
Project management plan includes features such as to-do lists, milestones, file sharing, messaging and notification, time scheduling and reporting, whiteboards, design templates, activity stream, white label branding and project forums. There are many more add-on features provided by basecamp and other third party vendors for more efficient project management, including billing tools, desktop widgets, iPhone app.
• Central Desktop: A popular project management service with a clean and elegant interface. 
Unlike other similar services, it provides different workspaces, depending upon the type of project, like blogs, wikis, forums etc. Features include chats, online notification, project notifications, reports etc. You can have up to two workspaces, 5 members in the free plan.
• Wrike: Another great online project management tool. Features include file sharing, Gnatt charts, 
history tracking, file import and export, project templates, MS Project importing, tasks etc. The best feature of Wrike is every hour backup which ensures that no data is lost in case of some issue. They only offer a free trial for limited time period at the time of writing.
• TeamBox: Teambox is not only a online project management tool but also a social
collaboration tool. It integrates popular social networks into project management. Its already integrated with Google Docs and Dropbox and has an iPhone and iPad app. Teambox is free to use for up to 3 projects.
Note that in the description of each service, I have listed only the free features available. Some of these services also provide additional features on paid upgrade, but I have not listed those features here as this post is about review of free project management services and software only.