I just found a free one called, GanttProject that looks interesting

and here is a list of some others that might be useful

by the way, are you familiar with http://techsoup.com ? I work for a non-profit company as well and we get most of our licenses for software through them. they sell donated licenses to non-profits. they don’t really sell them to non-profits, you just pay a small fee for each license. for instance, we only pay a $20 administrative fee for each license of MS Office 2007 Pro that we get from them. the catch is that you can only make so many purchases from them in a year and only a certain number of licenses at a time. just thought I’d share that with you in case you didn’t already know about it.