Command line interface and library for manipulating Apple ][ language files and disk images.
Applesauce - device for working with real floppies. The accompanying software is free and works with many kinds of disk images.
Call-A.P.P.L.E. - user group dating to 1978, offering books, software, magazine, archival documents, forums, etc.
CiderPress - long standing Windows software for manipulating disk images
Virtual II, AppleWin, MicroM8, MAME - emulators