GSplus is an open-source, cross-platform Apple IIGS emulator based on KEGS and GSPort. Despite still being in alpha, the latest build supports drag-and-dropping disk images onto the emulator to have them show up in the Finder desktop.
buckshot is an open-source, cross-platform image conversion utility. It takes modern image formats (PNG, JPEG, BMP, etc.) and converts them for use on the Apple II.
ksynthed is a small music editor and player library for Apple II, based on ksynth. The player library can be loaded into Applesoft BASIC, then CALLed to play songs (or notes).
Brock demonstrates all three tools in this YouTube video in which he uses GSplus, buckshot, and ksynthed to create an Apple II game called Applezini: