==> CSS Deluxe Quintopus
Share SIO devices between 2 computers.  The Deluxe Quintopus consists of a box
with two switched SIO ports and 4 unswitched SIO ports.

http://www.nleaudio.com/css/products/quintopus.htm

==> Supra MicroNet
Supports the connection of one SIO chain of peripherals to up to 8 computers.
When one computer accesses a peripheral device, the entire bus is occupied so
that the other computers on the "network" must wait.  The bus is freed five
seconds after a computer finishes interacting with the peripheral.

A printer/data buffer can make the MicroNet more practical.

Supra also provided a modified Atari DOS 2.5 that would re-try disk accesses
repeatedly in response to SIO timeouts.

http://www.atarimagazines.com/v4n10/productreviews.html

==> CSS Multiplexer ("MUX")
Description from the CSS online catalog:

The Multiplexer is a collection of cartridge interface boards that allow up to
8 Ataris to read and write to the same drives (typically a hard disk), access
the same printer(s), and talk to each other.  It is the first practical
networking system for the Atari 8-bit computer.

One "master" computer (any 8-bit) is equipped with the master Multiplexer
interface.  Then up to 8 "slave" computers hook up to this master, each having
their own slave interface.  The slave interface consists of a cartridge that
plugs into the cartridge port.  It has its own socket on the top so you can
use whatever cartridges you desire with the system.

The "common" peripherals (things that are to be shared) are connected to the
master.  On each slave, all disk and printer I/O is routed through the master
so no extra disk drives are needed.  The master computer can be configured in
any manner you wish.  You may have certain peripherals local to the slave or
routed to a different number on the master.  Note that serial ports (R: RS-232
interfaces) are not multiplexed.  All slaves are independent and do not need
to have the same program running on them.

http://www.nleaudio.com/css/products/multiplexer.htm

==> GameLink and GameLink II
This text by Andreas Koch:

In the late 80`s and early 90`s Chuck Steinman and Jeff Potter ("The ADGA
Group") developed some networking-computer-hardware to link two or more Ataris
together, so that multiplayer games are possible, where each user has its own
computer and tv/monitor screen. The hardware was/is computer independent and
will run fine on any Atari 8Bit computer (whereas most software for it will
only work on XL/XE computers). During a 3-4 year period of development two
different hardware add-ons were developed:

a) Gamelink-1: This hardware was developed in 1989/90. It links two
   computers together via the joystick ports. It is limited to a maximum
   of 2 computers and thus 2 or 6 players, meaning one free port per
   XL/XE computer and 3 free ports per 400/800 computer. However, the
   few existing games for this hardware merely support 2 players, no
   matter, which computer you have...

b) Gamelink-2: This hardware was developed in 1991/92. It links 2 to 8
   computers together via the SIO-port. One computer will then act as
   the master and has to boot up the software (from tape, disk, hard disk,
   etc.) first. Then all other "slave" computers connect to it and boot
   off of this master computer (one after another of course). In Europe
   we call this device "Multilink", mostly because of the games written
   by Bewesoft (Jiri Bernasek) called Multi-Dash, Multi-Race, Multi-Worms.
   A two-computer link-network can easily be done with one SIO cable, just
   open the end of the SIO cable and exchange cables number 3 and 5. You
   now have an easy two-computer (2-4 players) network-cable.

For some available software, that supports this networking-computer
hardware, see 8.16 which programs support networking computer hardware...

==> AT-Link (Alphasys)
Arianne Slaager writes:
I was actually surprised to read about the Gamelink-1, as I made a similar
cable myself, called the AT-Link.  This cable could also be used to
communicate with Commodore 64 computers, and I made driver software for both
systems at the time.  There were 2 drivers.  One as relocatable machine code,
and another as device driver.  Also in the package was a 2 player Battleships
type game where Side A had the Atari version, and Side B the Commodore 64
version.

...wasn't more than two old joystick cables in a crosslink configuration,
(Pin 1 and 2 linked to pin 3 and 4 of the other cable respectively) 

==> EightLink (Alphasys)
Arianne Slaager writes:
I also made a special high-speed Atari to Atari cable, called the EightLink.
This one was cartridge based system, with a PIA inside, which boasted a 8 bit
bidirectional, parallel databus, and a 4 bit crosslinked control bus. Transfer
speeds were such that two Atari's on opposite ends of a large hall could
transfer disk data faster than it could be read or written.  The actual cable
connecting the two was a flatcable with 33 leads, alternating ground and a
dataline across the width to minimise crossover disruption of data.  Also for
this link system, I made drivers both in relocatable code, as well as a device
driver.