Douglas Adams wrote three scripts for Doctor Who, and was also Doctor Who script editor for a year, during the late 1970s. All of the released videos of these three Adams-penned Who stories have since been deleted. However, an audio CD of Shada has also been released. Here's what you've missed.
Shada was never completed due to a strike at the BBC, and was released on video with a copy of the original script and Tom Baker's voiceovers to fill in the missing sections. This has now been deleted, but the order number was BBC48142 (VHS). It contains a number of plot elements which later turned up in Dirk Gently's Holistic Detective Agency. A Professor Chronitis, a time machine which is an office in a University, a certain joke, etc. The similarities are so great that for copyright reasons a novel of Shada could never be published. It was rereleased on the BBC website as audio with pictures (or 'webcast', as the BBC called it) and was released by Big Finish on audio CD.
Holistic Detective Agency also contains plot elements from City of Death, which Douglas also wrote. That story is credited to "David Agnew", which is a pseudonym used on BBC programmes where the writer's real name is not used for contractual reasons. The script was originally started by David Fisher, who couldn't finish it for personal reasons.
The Pirate Planet was only very recently deleted on video - there are several similarities between it and Hitchhiker's Guide, but nothing so great that Douglas might want to sue himself. Again, this was released by BBC video and has recently been deleted - the order number was BBCV5608.
Douglas also wrote a treatment (plot synopsis) for a film called Dr Who and the Krikketmen. It was never expanded to become a complete script, but most of the pertinent plot points reappeared in Life, the Universe and Everything. A look at ideas for this treatment can be found in Neil Gaiman's Don't Panic.