The small library of software available for the Macintosh platform has always been one of the principal hindrances to the growth of its market share (one of the others being overpriced hardware). In the late 80's and early 90's, with the library of PC applications and games booming, fans of the Macintosh were relegated to a seemingly smaller and smaller aisle in software stores. With PC developers reluctant to port their applications, what could be done to bridge the platforms? Add PC hardware to the Mac!
In 1987, Apple released its first ever expandable Macintosh computers -- the Macintosh SE and the Macintosh II. Taking advantage of this new opportunity to expand the Macintosh, AST Research introduced a pair of MS-DOS coprocessor cards for the Mac: the Mac86, an i8086 based card for the Macintosh SE and the higher performance Mac286, an i80286 based card for the Macintosh II. The introduction of PC hardware inside the Mac opened it up to the broad world of PC applications, however development of this card was dropped and in 1989 AST Research sold their technology to Orange Micro.
In 1994, the next iteration of DOS Compatibility Cards was released, this time by Apple, for their Quadra/Centris 610 series of computers. Called "Houdini" internally, the card featured a i80486 processor running at 33MHz. It was well received, but lacked some features needed for wide appeal, such as support for sound.
In February of 1995, after months of consumer anticipation, Apple Computer released the "Houdini II" DOS Compatibility Card for the Powermac 6100, which had a 66MHz 486 processor on a processor direct card that could easily be installed in the 6100. The package shipped with the latest Microsoft OSes at the time, Windows 3.11 and DOS 6.22, and could later run Windows versions up to Windows 98, although they weren't officially supported.
Using the Powermac 6100 with the DOS Card was really an elegant experience. The Mac OS and PC environment ran simultaneously and a hotkey could be configured to instantly toggle between the two environments. Alternately, a second monitor could be attached to the video out port on the DOS Card so
that both environments could be displayed at once, although I believe a hotkey was still necessary to toggle the keyboard and mouse over to the PC. The PC environment could also use the Macintosh's peripherals, including external modems, printers, network adapter and even the SCSI devices.
After the successful release of the Houdini II, Apple licensed the DOS Card technology to Reply Corperation who, along with Orange Micro, began to develop cards that were compatible with a wider range of Macs and were also more affordable.
Apple also began bundling the cards with some of its systems and marketing them as PC compatible. Systems that included a DOS Compatibility Card:
Sources:
Personal recollections, the houdini FAQ at
http://pw1.netcom.com/~sgulie/houdini_FAQ_v0.1, and the DOS Compatibility Card PDF available at
http://devworld.apple.com/techpubs/hardware/Developer_Notes/Peripherals/DOS_Compatibility_Card.pdf
Thanks to uberfetus for his help on this write-up. | 
