I believe that the first programming I ever did was on my TI-84 calculator in high school. I went quite a ways past the usual programs for computing the quadratic formula and such, and implemented such applications as a chemical equation balancer (using matrix operations, complete with string parsing and formatting), a solver for arbitrary law-of-sines and law-of-cosines problems (complete with reporting on the number of possible solutions), and step-by-step matrix transformation algorithms for row-echelon form and LU decomposition.
Unfortunately, the chemical equation balancer met a tragic end when I dropped my calculator in the laundry room and it reset the RAM. Everything else was backed up. You will always be remembered, BALANCEQ.