I am a frequent user of project management software, and have previously been a fan of open source Open Workbench. Onepoint Project have an open source version of their software, which IMHO is the best open source project management I have used. Their site can be found at http://www.onepoint.at/
Onepoint is a java app, which raises some portability issues for the purists. However, it appears to be a great candidate for the PortableApps treatment. I am using Onepoint regularly from my USB stick with no issues. It appears to be very portable natively. The visual look and feel of this app is streets ahead of the other open source project managers - and the software is robust and very functional. This app is highly recommended as a "must try" for anyone interested in project management software. It can import MS Project files.
I like this software so much, that after using the open source version, I have purchased one of their non-free versions.