Horse Armour. For many, that was the first time they would experience Downloadable Content. Well, if Boomtown are being entirely correct it was the first time many of them had heard of Micro-Transactional DLC that you had to pay for, but they think it's only fair that they start somewhere close to the beginning.
Oh No More Lemmings was probably the first 'expansion pack' that they ever owned. Requiring the original game to play, it was essentially extra levels for the classic Lemmings and was pretty well received. No-one ever questioned paying for extra levels, and no-one said that they should have been in the original game, yet apart from creating direct sequels, the expansion pack has continued to be a prevalent way of extending the original game and providing gamers with more of something they've enjoyed.