Gnaural is a free opensource binaural-beat generator released under the GNU General Public License.

Features include: tone and static noise control, manual frequency control (beat and base), infinite loop mode, a default meditation preset, a graph editor to easily create your own schedule and presets, ability to export audio into CD compatible wav and mp3 files, and other stuff.

The website currently has two additional presets for download, Deep Trance and Study Time.

You can download Gnaural and read more about it at it's website:
http://gnaural.sourceforge.net/

I've personally had some interesting results with the program, and a friend of mine who used it said he entered into a trance easier than he's ever been able to before. I have not used any other binaural generator before, so I cannot say how it compares. For those of you who have used other programs, I would be interested in hearing how this one stands up in your opinion.