The african-origin kalimba/mbira is another instrument that is small and cheap and easy to retune, and hence another good candidate as a gateway to microtonal music. Retuning is apparently common practice among the traditional Mbira masters in Africa - see e.g. this article by Joe Monzo on the MakeMicroMusic yahoo forum.
Reference material on Mbira
Paul F. Berliner, The Soul of Mbira, University of Chicago Press, 1993.
L. E. McNeil, S. Mitran, Vibrational Frequencies and Tunings of the African Mbira, Journal of the Acoustical Society of America, 2008.
Kalimbas used in microtonal music
Several members of the Yahoo tuning forums have used kalimbas for microtonal music:
http://launch.groups.yahoo.com/group/tuning/message/78060 (Kalimba tuned to harmonic series, George D. Secor)
http://launch.groups.yahoo.com/group/tuning/message/78106 (Kalimba tuned to Mavila pentatonic, Petr Pařízek)
There are YouTube videos of microtonal kalimba music:
Other microtonal kalimba/mbira references:
Further development of kalimba-type instruments
Kalimba-type instruments can also be developed further.