Vampire 30-Day Challenge : Favorite Supplement
Without repeating stuff I've already named in previous Day Challenges, I would then cite the Days of Fire online tickler as my favorite "Supplement" to the Vampire game. The online tickler was present on the www.white-wolf.com website way back when the game line was about to close with the Days of Judgement line of books. I actually "saved" the tickler (and added a few of my own entries) here cause back when it came out, I ran a series of end of the World of Darkness games for various groups and used the tickler to help them feel like their actions were making a difference (or at least getting noticed).
But yeah that was a lovely add-on to the game which really helped push the immersive quality and horror of the World of Darkness to the sessions. I am still excited to see more alternate forms of supplements that will come out to enhance games. Hunter, for example, had a series of modules called The Collection of Horrors which included audio clips to enhance the game. Some games like The Bell (Flying Monkeys) come with an awesome soundtrack to make the games more fun. I feel we can say goodbye to the days when only other published books count as supplements to the games we love.