ProjectLibre

Microsoft Project is a great tool, but what if someone sends you a Project file (e.g. .mpp) and you don't have the software to view it?  Unlike Word, Excel, PowerPoint and Visio, Microsoft does not provide a free viewer.  Well, you could nicely ask to the sender to resend it as a PDF or XLS file. Or you could use the free (I love free) Project Libre software and open it yourself.

http://www.projectlibre.org/