With Infocom's Steve Meretzky, Douglas wrote The Hitchhiker's Guide To The Galaxy text adventure game, as well as Bureaucracy.
When the game was first published, it was available in a box with instruction manual, orders for destruction (Earth and Arthur's house), a microscopic space fleet, some fluff, Joo-Janta 200 Peril-Sensitive Chromatic sunglasses, no tea ("Just like the tea professional hitchhikers don't carry!") and a Don't Panic badge. This version can still be bought from places like eBay quite easily at the time of writing - and this version is the only one that's really worth buying, as the game is legally available on the Internet (see below).
Infocom has since been bought by Activision, and none of their games are commercially available anymore. However, the game was rereleased as part of The Lost Treasures of Infocom and The Infocom Sci-Fi Collection. These collections are both out of print.
The Lost Treasures compilations haven't been made for quite a while now, and pretty much all stocks of these compilations have run dry (probably because of all the rec.games.int-fiction folk hunting the last few out!).
Originally, it had been planned to include it as a 'game within a game' as part of Starship Titanic. Sadly, due to "various strategic and copyright reasons", this did not happen. Here's a (rather old) quote from Tim Browse, of TDV:
"However, don't despair - you will see the Hitch Hiker game published again by TDV in the future… in a new and interesting form. More than that I can't say, I'm afraid (mostly cos we're not sure yet and are rather busy finishing Starship Titanic)."
This rerelease from TDV never materialised, sadly.
The two legal places that it can be found are the official Douglas Adams website, here and the BBC Radio 4 Hitchhiker's webpage, here.
The version on the Douglas Adams website is the original game with no additions or improvements. A mention of the possibility of a shareware version of the game was mentioned on the same page, but this has been removed (presumably because it was realised that nobody would pay to play a game which they could play for free).
The version on the BBC page is different from the original - you can play it with pictures illustrating the locations you find yourself in and the items you may (or may not) pick up. There are also parts which have been added to the game, supposedly.
The first online version of the game has no save function, but the one on the BBC page does. If you have a BBC account you get your own folder for your saved games - if you do not, you save into a public folder and have to remember all the names you give saved game files.
The InvisiClues? for the game are available online from the unofficial Infocom homepage, here. If you are really stuck, you might like to consult this walkthrough.
You don't need a Mac or a PC to play the games, nor do you need the right versions of the games for the right machines. Infocom games were interpreted, and hence the game files themselves are useable across all platforms with a suitable Infocom interpreter. The game file itself is still under copyright, but there are PD versions of suitable interpreters available for most machines, so it is quite possible to play HHGTTG on anything from UNIX to an Acorn by porting across a copy of the data file that you may legally own (or, admittedly, may have illegally downloaded), even if your copy of HHGTTG is for a completely different machine.
A good site for Infocom interpreters is the Interactive Fiction Archive. A visit to this page for a well-known interpreter might also help.
Infocom related discussions are abound in the rec.games.int-fiction newsgroup - it is possible to ask for interpreter recommendations (and indeed a lot of other things related to Interactive Fiction) there.
Richard Harris of The Digital Village adds:
"For your information, the Infocom games port very nicely to the Newton, using the YAZI Z interpreter written for it. A great way to pass the time on long flights."
The end sequence to the game mentions a second game, but this game doesn't exist. Douglas Adams started to write it, but because Bureaucracy had poor sales, Adams and Infocom dropped the project. Infocom tried to revive the project later, but the virtual death of Infocom in 1990 brought this idea to an end.
Another reason given for the fact Restaurant was never made was that Douglas Adams was too busy working on Dirk Gently and asked another person to help, but that person couldn't grasp the concept of interactive fiction and so it died the death.