Programming exercises

I just finished a nonogram solver. Any ideas for other small tasks that make good programming exercises?
Good web sites with programming exercises or competitions would also be relevant to my interests.

>>9
and some are a bitch
Exercise 5.51.  Develop a rudimentary implementation of Scheme in C (or some other low-level language of your choice) by translating the explicit-control evaluator of section 5.4 into C. In order to run this code you will need to also provide appropriate storage-allocation routines and other run-time support.

Exercise 5.52.  As a counterpoint to exercise 5.51, modify the compiler so that it compiles Scheme procedures into sequences of C instructions. Compile the metacircular evaluator of section 4.1 to produce a Scheme interpreter written in C.