The Apple IIGS was, and still is, the most impressive Apple II ever produced. They were introduced in September of 1986, and ran a much better processor, the Western Digital 16-bit 65C816 (compatible with 6502) running at 2.8 MHz. With third party hardware it can now go up to 18 MHz. It could also be slowed to 1 MHz for compatibility with Apple II programs. It included 256k RAM, and could be expanded to a staggering 8 MB. Aside from having 7 main expansion slots (lacking from the IIc), it also had a RAM card slot, modem and printer ports, ADB mouse port, AppleTalk support, both 3.5" and 5.25" floppy disk ports, an Esoniq sound chip, and an RGB monitor connector built-in. The IIGS display was a breakthrough in Apple II technology, displaying 640x200 pixels at up to 16 dithered (4 true) colors, and 320x200 at 16 true colors. Another advantage of the IIGS was that it could not only run ProDOS, but also the GS/OS, a GUI taking all the best qualities of the Mac OS, including the toolbox, which is similar to the Mac Toolbox. A IIGS is categorized by the ROM it uses, either 00,01, or 03. The 128k GS ROM 00 was included in the first IIGSes often with the limited edition "Woz" signature on the front. It was later upgraded to the ROM 01 which is needed to run the latest software. The next major revision, was the ROM 3. This included a larger 256k ROM, more expandable RAM, better support for the disabled, quieter sound support, different slot capabilities, and the ability to run GS/OS and GS/OS programs faster. Although a ROM 04 (code named Mark Twain) was in the late stages of production, it was never released to the public. These machines have a nostalgic quality for most programmers, including me, since I first learned to program in BASIC and Logo with these computers. The GS was finally discontinued in December of 1992, second only to the IIe in time on the market. Look at a screen shot of the IIGS ROM 01 boot, and the GS/OS System 6.01 desktop.

Code Names: Cortland, Phoenix (the project had been brought back to life after being killed), Rambo (when the design team was fighting for final approval from the executive staff), Gumby (from an impersonation done at the Apple Halloween parade).

Specifications:
Processor: Western Digital 16-bit 65816 at 2.8 MHz (up to 18 MHz using Transwarp GS or Zip GS).
Memory: came with 256k RAM, expandable to 8 MB.
Expansion: 7 expansion slots
Ports: modem and printer ports, AppleTalk support, ADB mouse port, 3.5" and 5.25" disk ports.
Display: up to 640x200 pixels in 4 true (16 dithered) colors, and 320x200 in 16 true colors.
Drives: supports Disk ][ and 3.5" 800k Sony floppy drives. Also supports hard drives. Although ProDOS can only handle 32MB, several users have reported IIGS HDs of several hundred MB.

Operating Systems:
ProDOS: all versions (I would assume), commonly ProDOS 8
GS/OS: all versions, commonly last version: System 6.01.

Models in this Series:
IIGS ROM 00: first system, 128k ROM, 256k RAM.
IIGS ROM 01: ran latest software, faster GS/OS operations.
IIGS ROM 3: improved GS/OS and GS/OS program operation, quieter sound, 256k ROM, 1 MB RAM, different slot capabilities.

On the market for: 6 years, 4 months

Interesting Facts:
If you have a IIGS ROM 3 the key stroke combination Command-Option-Control-N makes your IIGS shout "Apple II!". This is actually a sound file built into the ROM by the development team. Listen to a WAV file of the sound.

Five years after the introduction of the Apple IIGS, the 16-bit Super Nintendo Entertainment System used the same processor as the IIGS with a more powerful graphics co-processor. The original Nintendo Entertainment System was also designed to have similar graphics specifications as the upcoming IIGS in 1985.

Resources and Related Links:
The Apple IIGS entry at Glen Sanford's A History of Apple Computer.
The Apple IIGS entry at Richard Kilpatrick's now defunct "Apple Retrospective".
Apple IIGS Differences article at Apple's Tech Info Library.
The Apple II FAQ from comp.sys.apple2.
Picture from A History of Apple Computer.
Thanks go to HdwrNut for the info on the ROM 04, max IIGS speed, and max HD space corrections.
Thanks also go to GSer's corrections of the ROM 3 name, speed fix, "Apple II" easter egg info and WAV file, and other stuff.
Thanks lastly go to Eugene Iwanski, Jr. for his initial information on the "Apple II" easter egg.