Calendars can be customized by setting what days count as non-working days. However, I found Gantter.com's calendar tool easier and faster to use. Also, GanttProject lacks the ability to break plans down to an hourly schedule -- it can only be used to create plans that run on a daily schedule.

A user interface feature that I really like in GanttProject: Clicking on a task bar automatically highlights the corresponding task name in the sidebar window listing the tasks.

Unlike Gantter.com, GanttProject lets you print your plan charts to paper. It even features a very capable print preview tool. With it, you can perform tasks such as adjusting the width of your chart and clicking through different page sizes to ensure that your plan will be big, and legible, enough to read as a hard copy.

Native Microsoft Project files can be imported into GanttProject -- at least in theory. In practice, a couple of test files in Microsoft's MPP format I tried failed to import correctly or could not be imported at all. Files in the MPX format fared better.

GanttProject loads and saves files in its own proprietary format, along with importing and exporting files as XML. It can also export plans to spreadsheet format in CSV format ("comma-separated values").

One thing that sets it apart from other free applications is that GanttProject can create PERT charts, where the tasks in your plan are depicted as a series of interconnected boxes. You can drag and re-position the boxes to other areas on the timeline, and the pathway lines between it and other task boxes will adjust accordingly. This works well as a quick and easy way to fiddle around with your plan.

jxProject

JxProject is another Java application. According to its creator, Peter Hawkins, it was borne from his frustration with other software that he felt was difficult to use when dealing with resources that were part-time or located in different time zones. Thus, although many project management applications don't support time intervals of less than an hour, jxProject lets you plan projects with tasks that run for just a few minutes.

jxProject has an interesting "resource optimizer" tool that can analyze your project; figure out which parts, for example, have unusually long periods of time devoted to certain tasks; and estimate which tasks can have their priority adjusted to be scheduled earlier. As a result, this tool can help to compress/shorten the overall life cycle of your project. Practically speaking, the optimizer works best if you're intentionally including plenty of time for your project but would like to see how much sooner tasks could possibly be completed if you take a "crunch time" route.