One folding style with several printable designs (some takes lots of ink). Creator also offers OpenOffice.org Draw templates (Creative-Commons license) to create your own Letterfu design.
P. Bihler, M. Fotsing, G. Kniesel, and C. Joffroy. iiWAS '08: Proceedings of the 10th International Conference on Information Integration and Web-based Applications & Services, Linz, Austria, page 677-680. (2008)
A. Blandford, P. Curzon, J. Hyde, and G. Papatzanis. Interactive Systems: Design, Specification, and Verification, 15th International Workshop, DSV-IS 2008, Kingston, Canada, (2008)