I spend a lot of time listening to music, so I built my own music player for my large collection of MP3s. I'm not a big fan of streaming and could never find a player that did all the things I wanted, so I figured it'd be best to build my own using Web technologies. It's been fun adding each feature -- automatic playlists with intricate rules, some smart shuffle features, stats, tag management, displaying waveforms, syncing with my phone, etc. -- and I've learned a lot on the way. I've also fine-tuned the UX to my personal preference. It helps when you only have to worry about the exact set of features that you want!