My favorite would have to be Airborn by Kenneth Oppel. (It also has two excellent sequels: Skybreaker and Starclimber.) The story unfolds 800 feet in the air on one of the luxury airships that dominate the skies in the Victorian-age setting. The airship Aurora is kind of the Titanic of the skies but the story is very much angled at a Treasure Island-like adventure. In addition, the traditional steampunk inventions (particularly the flying variety), air pirates, strange creatures, a strong plot, and an excellent hero and heroine team create quite a story. And that's only in the first novel...!

(Why, yes, it does have a website!)

(*Edit: Go, link, go!)