Emulators and Tools
From 8BitDev.org - Atari 7800 Development Wiki
Emulators and Tools
MAME
MAME is the emulator with the most hardware-accurate 7800 emulation at this point in time. For this reason its the recommended platform for developers. (in addition to real hardware, of course) With a less accurate emulator, it's easy to create games that won't be able to run on real hardware.
You can find the latest binaries at MAMEDEV.ORG
7800 Utility Cart
The 7800UtilityCart is a 7800 ROM that will display information about your 7800, provide joystick test, display color bars, test internal and external RAM, test sound chips, and manage your AtariVox and HSC devices.
The 7800UtilityCart is copyright 2015 Mike Saarna, and is currently in beta status.