Choose wisely.
Expression evaluator: (ie: 9+4*sin(37)^2)
HTML Escaper (see ideas thread for details
Substring Finder
Text File Compressor
Data File Compressor
Tetris
Source code translator (difficult, see ideas thread)
a LISP interpreter (see previous poll thread for details)
Source code beautifier
Nibbles game clone
HTML analyser (see ideas thread for details)
Quiz Program
M3U (mp3 playlist file) editor
A basic drawing program
A Slot Machine (aka fruit machine)