I didn't want to post this with specific threads on specific converters. It needs to stand alone (and maybe get stickied as it is something that will be on-going?).
There are three system/apps out there to convert Midi to ABC. They each have strengths and weaknesses. We tend to use Maestro, as the interface is very easy to work with. Lots of people thing Firefern is the best, and certainly it can handle large numbers of tracks. It is not as simple to learn or use though. Hardest to use (in my opinion) is BruTe, which is still in alpha testing, but available to try. You need to specify instrument and drum mapping in text files, and then run the associated programs to convert. No real GUI (a basic editor for the mapping files is available).
In the interests of which works best, I'll be posting comparatives here, and linking in things as needed.
Here is an interesting experiment, using a four-part tune, Take 5. To demonstrate the state of his project, the author of BruTe ran the same midi through all three converters, using the defaults and not editing the input midi or the resulting ABC. This folder in my dropbox contains the three results, and a text README file explaining it. Grab this HERE and let your ears judge for themselves.
Here are links to the three systems:
Maestro Project Page
Firefern's Web-based Conveter
BruTe Code Distro (alpha)