A little known fact: the saxophone family was originally comprised of two sets. One, which is by far the most common today, was developed for use in wind bands, and is in the keys of Eb and Bb. The other is in C and F, and was intended to be used in orchestras.
The orchestral instruments never really caught on, so nowadays nobody uses the C or F, except for a handful of old C instruments. They're quite handy, since you don't need to worry about transposition - you play a C, and it comes out a C. By contrast, on a tenor sax in Bb, you play a C, and it comes out as a Bb.
Both of these families included everything from bass up to sopranino. I'm fairly certain that contrabass saxophones were not part of Mr. Sax's original plan, though they were added later - they're still quite rare. I've never had the good fortune of playing one, but the bass saxes I've tried have been quite a rush! Though not 7 feet tall, as stated, their 4 feet of very large tubing, and massive bell makes for quite an intimidating machine!
Unfortunately, the sub-octo-contrabass never *really* existed. Some ridiculously huge saxophones were built as promotions by factories, among them Conn, but they were never intended to be played, being simply a larger version of the other saxes - the keywork was not functional.
These days, saxes are used all over - not as much in popular music as they used to, and not as much in classical music as classical saxophonists would like, but they remain the main wind instrument in jazz, and can be heard occasionally in just about every other type of music in existence.
This is primarily due to the instrument's incredible versatility: a wide range of dynamics and timbres, from fuzzy whispers of sound to strident screeches, honks and blasts to lush silky sounds that put any violinist to shame. As well, a good player has the ability to manipulate the pitch immensely, making for some fantastic special effects. Indian musicians have begun using the saxophone in place of, or alongside, more traditional instruments, exploiting this aspect of it.
As well as all of these intriguing effects, saxophones can also play very quickly and with great agility, compared to similarly flexible instruments.
In short, the saxophone can be used just about anywhere, if the saxophonist has the technique, and the understanding of what he/she is doing. It's not a surprise that the instrument has become so well-known, and such a familiar cultural symbol in the hands of everyone from Lisa Simpson to Bill Clinton.