http://january.cc/

Even though it's not january yet, it sure fits the season. January is a generative music tool created by Disasterpeace. For those unfamilliar with Disasterpeace: he did the music for Fez, among others. 

This music tool has you walking around on a small, pixelated landscape. You can walk around and lick falling snowflakes to create lovely, melancholic chiptune music. Different snowflakes make different sounds, some are single notes, some chords and others change the key. The controls are detailed below the game. Be prepared to lose a good couple of hours to this gem.