ABC fies are simple text files, so any text editor can be used to write and edit ABC music. Of course, specific softwares are available to edit ABC music, convert it into scores and play its audio content.
If you’re on iOS, of course, there’s Nota ABC, which hosts this small wiki; there are also other apps, just check on the App Store.
Needless to say many apps are available for Android on the Google Play Store.
If you’re after a web based option, check out Michael Eskin’s tools.
If you’re a developer and want to add ABC music to your website, check out Abcjs, which incidentally power this very website.
There are tenths of applications and command line tools to manage ABC music; the ABC Notation websites features the most comprehensive list. Check it out!
On the other hand, if you’re looking for ABC tunes, I suggest you visit The Session, which features one of the most comprehensive collections of user-sumbmitted tunes.