The Tetris music from the C64 Tetris game, recorded on a real C64. I converted the SID file to a CRT file, and then played it with my DIY cartridge on a C64. Listen at least to the first 3 minutes, the changing time signatures, chord sequences and different keys are awesome. My favorite part starts at minute 9:30.
You can download the original game from various websites like CSDb and then play it with VICE, which of course, doesn't sound as good as on the real C64.