Music for Super Mario World hacks
ROM hacking (Super Mario World to be exact) has been a major influence on my creative skills and output.
Music is another thing you can customize in your SMW ROM hack with relative ease. I tried my hand at it when the technology was still more or less in its early days, but it's never been my strong point. Maybe I just don't have the patience or the attention span for the fine-tuning that's required for SNES music to really sound good.
I've submitted a handful of songs SMW Central for others to use. (The submission dates on the site aren't always accurate since many of them were reuploaded years after they were made.) I'm listing only custom compositions here, not ports of existing songs to the SNES format.