What are the Naruto hand seals based on? There are a number of hand seals in Naruto that are used to perform jutsus. I know that the seals themselves have different zodiac signs. Is there any lore or historical basis for these seals, or was it invented by the creators of Naruto?
The hand seals in Naruto are based in Mudra, which are symbolic/ritual gestures associated to Hinduism and Buddhism. Mudra are used in everyday life, from religious practices to dances, going through martial arts as well. The most well-known Mudra is probably Añjali Mudrā, which often accompanies the greeting Namaste.
In the French version of the Mudra Wikipedia page, they do indeed link directly to Naruto's hand seals page, unlike what happens in the English page. However, they don't add much info other than the fact that they are called Mudra, and that they're based on the Chinese Zodiac.

From Wikipedia's page on Mudra, on its use on Martial Arts (highlight by me):  


  One of the more curious things that I encountered in my martial arts training was the use of mudra in combative arts. Mudra (Japanese: in), for those who aren't familiar with them, are these weird hand gestures that are derived from esoteric Buddhism (mikkyo), particularly the Tendai and Shingon sects. These gestures are supposed to generate spiritual focus and power which then are manifested in some way externally.  


However, as far as I know (and also according to this page) the use of Mudra in Ninjutsu is made up by Kishimoto (once again, highlights by me):  


  Although the use of hand seals in ninjutsu is entirely made up by Kishimoto, seals are used as ways to focus energy during Buddhist meditation. Also, special hand signs, called mudras, are used to convey a certain concept into the user. For example, a right hand raised with palm facing outwards means “no fear”, and grants the user that concept’s blessing through their meditation. The seals in Naruto are from the oriental zodiac, the 12 animals which name the years (I was born in the year of the snake, for instance), the properties of each seal representing what that animal’s qualities are - Eg, Tora/Tiger seal is fire.


Basically, in real life hand seals are used in meditative practice, and are supposed to help the user get into a certain state of mind.
In the Naruto universe, Kishimoto decided to use these ancient signs in a different way (associated to Ninjutsu), but their purpose is basically the same. The user performs hand seals in order to gather/mold chakra, which means that he is entering a certain state of mind when he does so. Also, when a user masters a jutsu he needs to perform less/no signs before he can do it, which means that the meditation phase before the use of the jutsu can be skipped/sped up. Similarly, the more one meditates, the easier it gets for him to get into a state of meditation.
So they are basically based in an existing concept, but used in a different context (in Ninjutsu), with basically the same function.

For more info in Mudra, also see this page.