I get asked this question a lot since people are working with different OSs and they want to use somewhat familiar applications to what they are used to. The main answer is that I have found two replacements for MS Project
Both of these replacements are open source and essentially free. The first one is a product you can get when you use Linux. the name of this product is Planner. You might have to install it using the package management utility(add/remove programs.) It provide similar views and functionality to MS Project. Part of the reason for this tool being here is so that there is no whining when a company decides to go Linux full time.
The next tool is called OpenProject. This one might be the one you might want to take a good long look at. It is available in all three of the Major OSs, Windows, Mac OSX, and even Linux. In fact they provide two packages, rpm and deb. I ran across this one while seeing if there were any open source project management software, and this one popped up.

I have only managed with MS Project because of the limitations of my contracts with companies. I have tried working a fictitious plans with both products and I can say that they work well, and seem very stable. That’s hardly a recommendation, I know, but if you are looking for a replacement for MS Project, then you might want to check out these two.
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