Little Sound DJ, or LSDJ, is a tracker program written for the Game Boy/Game Boy Color platform by Johan Kotlinski (aka Role Model). LSDJ v.1.0 was released on January 22, 2001 and has been steadily updated by Kotlinski as users request more features and as bug fixes are needed (at the time of this writing, the current version is 3.0.7). The ROM image can be purchased and then downloaded for $39 USD (once purchased, all updates are free), and there is also a free demo version which lacks a save feature.

The following text is reproduced here without permission of the copyright holder. The original author is Johan Kotlinski and this text was copied directly from www.littlesounddj.com:

CONCEPT
The basic idea is to transform a plain Game Boy/Game Boy Color into a full-fledged music workstation.
Sequencer
The sequencer of Little Sound Dj has a very open structure. It was designed to leave all the musical possibilities of the Gameboy in the hands of the user. The system can be said to be both simple and complex; it has a slight learning curve, but when learned, it is possible to transcribe a complete Bach piece to it on less than one hour.
Sound
The Gameboy sound chip offers four channels with 4-bit sound. Custom waveforms can be created by free-hand drawing, or by using a subtractive synthesizer with resonant filters. For extra versatility, Little Sound Dj also contains a quite powerful arpeggiator, which possibilities go far beyond the classic C64-style chords.
Samples
Sample-wise, Little Sound Dj boasts a set of 59 phonems for programmable speech. Besides that, it also features drum kits sampled from machines TR-606, TR-707, TR-727, TR-808, TR-909, CR-78, CR-8000, KR-55, DR-55, DR-110, DMX, Drumulator, RhythmAce, TOM and LinnDrum.
Synchronization
It is possible to use a link cable to sync two Game Boys; great for party fun or for added polyphony! If you want to, you can sync LSDJ with Nanoloop. MIDI sync is possible too, if you are ready to build your own LSDj Midi Interface.
The Interface
The workspace is divided into ten screens: song, chain, pattern, instrument, table, groove, project, frame, synth, and word. You can move through the different screens by holding select and pressing the direction of the screen (a map in the bottom right corner shows the current screen and its physical relationship to the others). The song screen has one column for each of the four tracks in which you enter chains. In the chain screen, you can chain together patterns, and in the pattern screen you can activate instruments with the desired note (octaves 3 – 8) and also add effects (legato, sweep, pitch bend, and others). From the instrument screen you can make an instrument that is either a pulse, wave, noise, kit (sampled sounds), or speech (sampled phonemes) instrument, and then set the various properties of the instrument including envelope, sweep, and which, if any, table to apply. The table screen is four columns of 16 rows which allow various commands to be applied to the instrument being played, allowing you to create interesting effects such as arpeggios. The groove screen helps set your pace by letting you set how many ticks (an abstract span of time about 1/60th of a second) each note is to be played for. You can also set up asynchronous amounts such as 3 ticks for the first note, and 4 ticks for the second to get a swinging rhythm. The project screen has the master controls, allowing you to do things like set the tempo, save your song, purge all memory, and synch your Game Boy with another. The frame and synth screens are used to generate waveforms to be used in the wave channel. The word screen is a hidden screen which only appears when using the speech instrument, and it allows you to chain together phonemes to make words.

Getting a Cartridge
LSDJ cartridges are very tough to get your hands on these days. The Game Boy Advance has shifted interest away from Game Boy/Game Boy Color and so it has become increasingly difficult to find the hardware needed to put the LSDJ ROM image onto a Game Boy cartridge. What you need is a transferrer, a 8 MB cartridge (4MB will do for any version earlier than 3.0) that is compatible with your transferrer, a copy of the LSDJ ROM image, and a PC with a serial port (I believe all transferrers connect to serial ports). Of course, you can always use the LSDJ Demo ROM image on your computer with the Game Boy emulator of your choice (most emulators have a save feature which you can use to circumvent the demo ROM's lack of a save feature), but many argue the original Game Boy hardware produces a sound that the emulators just can’t produce quite right. Also, one of the greatest things about using the Game Boy as a tracker is the ability to carry it with you on the subway.

LSDJ Live
A feature definitely worth mentioning is the live mode. When in the song screen, by holding the b button and pressing the direction pad, live mode will be engaged. Typically when in the song screen, pressing start will begin the song you have laid out, but while in live mode, you can choose which tracks to play, and at which point. This allows for some very interesting improvisation. If you are in New York City or nearby, make sure to come see Joshua Davis (aka Bit Shifter, www.bitshifter.cc) at his next show if you want a good example of just how intense the LSDJ live mode can get. There also is a PC keyboard synchronization mode that allows for some more interesting improvisation techniques for live shows. I recently saw Jeremiah Johnson (aka Nullsleep, nullsleep.8bitpeoples.com) play a set with a standard PC keyboard strapped over his shoulder, and it was one the greatest and geekiest things I’ve ever experienced. These features are a terrific boon to LSDJ artists who perform live.

Sources
http://www.littlesounddj.com
http://groups.yahoo.com/group/lsdj