Korg DS-10

The Korg DS-10 is a synthesizer cartridge for the Nintendo DS, that looks very much like the 1970's-80's Korg synths.
It's a closed system with a Song mode, 2 realtime Synths and 4 sampled Synths (Drums).

Pros:
The Synth is basic, nevertheless you can make amazing sounds.
The 2-screen interface provides for a fast workflow.
There are much Automation and Patching possibilities.
The LFO can be as slow as nearly 5 minutes.
There's a KAOSS Pad for entering Notes and Automation.

Cons:
No Wave file export and no MIDI.
Loading a Synth Patch can destroy the Pattern Sequence. Be careful using it.

Save your Session regularly before making big changes.
These pages deal with features that can not be found in the manual.

The developers made good use of the 2 screens of the Nintendo DS ("Double Screen").
- one screen is used as an overview screen to select the item you wish to edit
- the other screen is used to show and edit the selected item

Each Session contains 16 Patterns. Each Pattern supports 2 Realtime Synths for every Pattern and 4 Sample Players for global "Drum" samples. The Drum samples are created using the same Synth as the realtime Synth. When you're done the sounds get converted to static samples for the "Drums". You are not limited to Drum sounds of course. You can also create Sound effects, Synth sounds, it does not matter.

You can save a Session, and you can save an individual Synth Patch, which you can Load into a Synth when needed.

You can wirelessly clock/start-synchronise 8 different Nintendo DS machines with the Korg DS-10 software.
Otherwise, there are no sync posibilities, outside of the Nintendo DS.
Or be brave, turn all of the music to one of the stereo channels, and and use the oher channel for a sync beep tone.

Some amazing examples of DS-10 use:
Anthony Seeha: KORG DS-10 PLUS: 'Live Mixing (Artist Showcase)'
Toraoka Soja: KORG DS-10 Ridge Racer (Ver.1.1-Release)
Kenichiro Ohishi: Stylophone+DS-10 - The Girl From Ipanema
NATT021: YMO "TECHNOPOLIS"
NATT021: YMO "Firecracker"
NATT021: YMO "RYDEEN"
NATT021: Ryuichi Sakamoto "Merry Christmas Mr.Lawrence" (live)", (studio version)
kskproject: dennki groove NIJI DS-10 REMIX

Manual
The Manual has been scanned, and can be found at the Internet Archive.
https://archive.org/details/korg-ds-10-manual/korg_ds-10_manual/
Visual tutorial from developer Nobuyoshi Sano: KORG DS-10 #002

Pretty good reviews with great examples
Bad Gear: Nintendo Korg DS-10 - Game Boy MS-20???
inside synthesis: Review: Korg DS-10 Analog Synth Emulator for Nintendo DS