Overview
In 1979, Mattel Electronics released a system which they hoped would take over the console market from the then-leader, Atari's 2600. Intended to be more than just a console, it was planned to be expandable, with a keyboard component advertised and eventually launched (very poorly with a tiny distribution, most later being bought back by Mattel). Advertising campaigns comparing the superior graphics of Intellivision to the 2600 helped shift around 3 million "Master Components" up until the system's death in 1990 - making it almost as long-running a machine as the 2600.

Models
There were three models of the Intellivison Master Component. Like just about every console which went through a remodel during its lifetime, the 'guts' of the machine remained the same, with only cosmetic differences separating them. However, one key difference was that, unless the circuit board was modified by the user (unlikely), the original Intellivision could not use the 2600 "System Changer" module, which allowed Atari games to be played through the Intellivision.

The Intellivision II sported a new, leaner look and had detachable controllers. It also featured a power adaptor which is completely impossible to replace - so if you have one, hang onto it (and if you're buying, don't get one without it). The Intellivision III was never released, but the "INTV III" was released by a company which had bought the rights to the Intellivision when Mattel Electronics broke up in 1984. This was essentially a remodeled Intellivision I though, and could not use the System Changer.

Technical Specifications
CPU: A General Instruments 16-bit CP1610, clocked at 894Khz - not even a full 1Mhz!
RAM: 64k address space
Maximum colours: 16
Maximum sprites: 64
Sprite size: Maximum of 8x8. (The Intellivision had some sprites built into ROM which could be called by programs - the famous "Intellivision man" is one)
Sound Channels: 3, plus one noise generator
Resolution: 192x160

Accessories
Controllers: These features a disc which one turned to move the player on-screen, and a keypad. Over the keypad an overlay could be placed which was specific to each game, labelling different functions. Games without them are worth far less than those which are complete in today's market. They were non-detachable on the original Master Component.
Intellivoice: An interesting addon, which provided voice for some games. Only a few required it, but it was nice nonetheless.
Keyboard component: Oh dear. This was delayed time and time again, and by the time it was finally released, it only was given a tiny regional distribution. It never went on sale nationally or internationally and very few games were ever made for it, let alone released.
Entertainment Computer System: The keyboard's replacement. Again using a non-standard power supply (Those crazy Mattel guys!), it allowed programming in BASIC. Upgrades such as a printer were planned but never developed
Music Synthesizer: An upgrade for the Entertainment Computer System, this was a 49-key piano-like keyboard which was compatible with... one game.
System Changer: Ah, here we are. This device plugged into an Intellivision II and allowed for Atari 2600 Cartridges to be plugged into it for play through the Master Component, essentially boosting the library available for Intellivision users to larger than the Atari's! Mattel were sued by Atari, but won due to the fact that nothing within the 2600 was custom-built - all the parts were 'off-the-shelf'.