As a HeroQuest enthousiast, I've browsed the interwebs a lot in search of resources to add to my HeroQuest game. There are many, many sites out there, but few as complete as Phoenix's.
His site has all the original quests from the base game and all the addons, in handy PDF format.
You can also find the unofficial HQ quests that were published in magazines and such.
On top of that, he hosts many Fan-made quests as well: Single Quests, Mini-Quest Packs, Quest Packs and Contest-winning quests.
This is certainly a site to visit if you need HeroQuest material!