I've spammed everything else with this, so why not here too... I wrote a tracker for the BBC Micro, and did a cover of Captain's Space Debris
It's my first significant piece of 6502 machine code. It turns out to be surprisingly easy to work with --- nicer than the Z80, I'd say, but mainly because there are so many fewer choices that there's only really one sensible way to do anything, unlike the Z80 where you're constantly struggling to fit everything in registers. The 6502's index operations make so many things easier, too; the tracker's full of loops where I'm doing note or tone lookups, and this becomes almost effortless. It'd be a different matter if any of my lookup tables where bigger than 256 bytes, of course.