There are actually two of them available, one that only works on pc windows, the other that should work on any platform that supports java.
Let's talk about the first one, written by KidQuaalude. This one works on pc windows only (from windows 95 to 7, I assume). Program loads up a .sav file and shows you the songs and the instruments that are in the .sav file. You can save any of the songs and instruments to your pc as individual files and, of course, you can load them up as well. Remember that a .sav file for the Korg DS-10 cart contains 18 songs and 24 synth patches.
KidQuaalude Korg DS-10 Save Tool
See the thread where it all started on the DS-10 forum:
[Windows] Korg DS-10 Save Tool
Now, since not everybody uses pc windows, somebody had to come up with something that would work on linux and/or mac (actually it works on everything that can run java). Well, here comes QauNuckShin (I think it's Swedish). It's pretty much the same idea as above so here comes the link:
Korg DS-10 Save Manager Tool
And, to be complete, here's the thread on the DS-10 forum:
[Multiplatform] Korg DS-10 Manager
With both of these, you need to be able to transfer the .sav file (saved data) from the Nintendo DS to you computer. One way of doing that is to use the NDS Adaptor Plus.
For both of these amazing tools, make sure you backup everything before manipulating your precious save files.
Now, what I really would like to have is something that exports a DS-10 song to a midi file. Would just need, for each channel and each note, the pitch, duration and velocity (volume). Don't care about the instruments, really. But, for that, I would really need to know the format of the Korg DS-10 .sav file (which is kinda why I am quite amazed/perplexed at how those two guys were able to write those tools). If you know, let me know! That would be a dream come true to be able to load a DS-10 track into a DAW like FL Studio, Reaper, Ableton Live, etc. Imagine being able to roughly compose a tune on the DS-10 and finish it up on a DAW. The best of both worlds!