I was thinking the other day about which websites might still exist in 100 years on whatever the web becomes, and Wikipedia to me seemed the most likely. It manages to be both stable and flexible. Functionally it's almost identical in all important ways to how it was ~25 years ago, but organisationally it has adapted to handle the incredible increase in scale. It is certainly flawed, because it's made by humans, but I think it may be the web's greatest achievement.