There have actually been quite a number over time, some mentioned above. Most major manufacturers have done one or more shrinks. The A319 is probably the most successful of all, but the 737-500 also sold in good numbers. Both types had good secondhand value as well.
I think a BAC One-Eleven prototype was unique in being physically cut and lengthened to be the prototype series 500, and then the same airframe was again cut and shortened back to be the prototype series 475.