Any sort of code is useful. You can tell a lot just from reading a code sample: you get a sense of the candidate's attention to detail, coding habits, desire for simplicity, preferred coding standard, technology background, API design skills, knowledge of standard library functions, and knowledge of design patterns and idioms.

Try to keep things short & well-written. An interviewer is not going to want to pore over a 20 KLOC project looking for something cool. A small, well-defined library that does something useful is best. Like a useful utility class or small project that does something cool.