A PHP IDE I wrote in C++ when I was 16 years old. It was my first practical exercise use C++, from which I've learned a lot.
A C++ library I made because I wanted to parse various file formats and did not want to use a heavy library for each of them. It was made as modular as I could think of at the time. Looking back with the lessons I've learned by reading and programming in Python, I would have handled it differently. Still, it was a useful exercise.