Digimon Adventure the OVA (original video animation) was released in Japan in March 1999, the day before the first episode of the first season.
It's not really known in the West in its original format however, as it was cannibalised into the (utterly atrocious) Digimon: The Movie, but as a standalone undubbed OVA it's a very enjoyable start to the Digimon anime series, and is an extremely important part of the Digimon Adventure canon.
The OVA gives us far more questions than answers, and some of these don't get answered for a good few years.( Cut for lengthy review and spoilers )
All in all though it's a very strong start to this branch of the franchise, so I give it 8/10!!