"ProjectLibre is the leading open source alternative to Microsoft Project. It has been downloaded over 2,700,000 times in over 200 countries and has won InfoWorld "Best of Open Source" award. ProjectLibre is compatible with Microsoft Project 2003, 2007 and 2010 files. You can simply open them on Linux, Mac OS or Windows."
I've just started using Project Libre. It's a great and useful program. I'd love to see a portable version.
(I searched and didn't find this requested here before, but if it's here and I just missed it please delete this post.)