About 3 years ago, I evaluated various open source and/or free Project Management software packages with an eye to finding one that could produce project plans and Gantt charts for free. At the time, the two leading contenders were Gantt Project and something called OpenProj. I chose GanttProject as the most mature product at that time. I also liked the fact that it was cross-platform since I was working in both Windows and OSX, and I hoped to move into Linux eventually. I suspect that in the intervening several years, GanttProject has only gotten better. A quick look into the status of both OpenProj and Workbench suggests that the current status of both of those is unclear or stagnated. By contrast, GanttProject seems to be in for the long haul, and so may be worth checking out.
Note that I didn't actually use GanttProject for ongoing project management: I just used it to create a Gantt chart for my team and supervisors, and then again to produce updated versions of the workplan and an accompanying Gantt chart every 3 months or so. For those purposes it was pretty good: I was using a beta version, and there were a couple bugs but I found workarounds for those at the time. It looks like they have an official release out now. I can't speak to whether it would be any good at whatever you may need for ongoing day-to-day project management, as I don't know what that set of needs entails.
I've just spent a few minutes on the GanttProject website now, and I see that they are in the midst of beta/alpha-testing a new version (a major update, not an incremental update). If you are just getting started on your projects right now, then I would be inclined to investigate the latest alpha/beta version instead of using the latest stable version. I'm going to download and install it myself now just to check it out. Links to that alpha/beta version are available here:http://ganttproject.blogspot.com/2011/0 ... d-306.html
[Later: I installed the alpha version of the "praha" build, and I must say that it is not really very user-friendly. If you are okay with that (as I am) then that will be fine for you, but if not, then you're probably best to stick with the latest stable version. If you go for the alpha version I linked to, then you may want to download the stable version anyways, if only so you can extract the icon files from the exe, since the alpha version doesn't include any icons. If all that seems like an awful lot of work just to test something out, then the alpha version is not for you, just use the regular version and I'm sure it installs just fine like any other Windows software.]