MS Project Server was created to allow people share their MS Project files on the Web. However, MS Project Server does not make collaboration on the project plans effective. It seriously limits team members' ability to contribute to the project plans, putting the burden of updating plans on the manager's shoulders.
Besides giving 24/7 access to the project plans, a good MS Project Server alternative should allow your team members to collaborate seamlessly on those plans. People should have the ability to share knowledge, discuss project issues, reassign tasks to each other and reschedule tasks if necessary. It is great if other team members receive alerts about the changes, so the entire teams stays on the same page and is able to react to the changes quickly.
Unlike MS Project Server, online project management software is often integrated with e-mail. This allows you to stay in your familiar environment. Plus, this saves you time on creating and updating tasks.