A little while back I looked for alternatives to MS Project for a non-profit organization. The only one I found that was worth getting was IMSI TurboProject. At $99 it provides most of the capabilities of MS Project. I tested it by defining a relatively complex project. It has its own quirks, but is quite capable and exceeds the capabilities of Project in some areas. It can import but not export Project files.
If you step up to the Pro version you are paying very close to the same as for MS Project, but you can swap files with MS Project users both coming and going, and it claims to have multiple project support. I have not tried the Pro version, so I cannot speak from experience about the enhancements over the base version.