I'll bet you didn't even know there was such a thing as a guitar built by Stradivarius. The late 17th and early 18th centuries were a particular golden era for the guitar and it was a hugely popular instrument in the royal courts of Europe so it was inevitable he would have built some. There are four guitars by Stradivarius still in existence and here is an excellent website with lots of photos.
There are some modern copies that are played by performers specializing in historical performance, but the Stradivarius guitars occupy a tiny position in the modern musical world, compared to their violin and cello cousins. Why is that?
Here is an interesting thing about guitars and violins. The violin top (and bottom, though that is less important) is vaulted as you can probably see in this photo of a Stradivarius violin:
This gives the top a great deal of structural integrity, making for a long life. The top of the guitar, however, is flat. This top, moreover, is twisted by the tension from the bridge. In a modern instrument this amounts to some 125 lbs of torque. After twenty to thirty years, the acoustic integrity of the top starts to go and significant loss of sound is the result.
But the vaulted top of a violin, with a different sort of bridge entirely and supported internally with a bass bar, is far stronger.
True, the guitar also has internal strutting, which does help, but the flat top makes for a short lifespan compared to the violin.
UPDATE [Oct. 28, 2012]: This post keeps being popular so I am updating it by including a musical clip of Baroque guitar. Here are the first four movements of the best-known suite by Robert de Visée (1655 - 1733) including the Prelude, Allemande, Courante and Sarabande. The guitarist is Rob MacKillop and the guitar looks quite a bit like a reproduction of a Stradivarius.