Sign Languages for the Deaf. A method of communicating using hand, body and facial gestures. It is a visual language. 
There are different forms http://www.ethnologue.com/ethno_docs/special.asp lists 114 sign languages. In the same way that spoken language have evolved Sign Languages are a product of their Culture. Deaf people (who use sign language) should be seen as a linguistic group rather than people with a disability.  Some have signs and methods in common, for instance both American and Irish Sign Language use one hand for fingerspelling. Whereas British Sign Language uses two hands for fingerspelling. 

There is an International Sign Language, which has drawn vocabulary signs from different sign languages that Deaf people agreed to use at international events and meeting. It is an artificial language. A further explanation of International Sign can be found at http://www.handspeak.com/byte/isl/info.html

Sign Language is not a mimicry of the local spoken language; they each have their own unique grammar, just as do spoken languages. For instance in spoken English you would say "What is your name?" but in British Sign Language you would sign "Name (you) what?".  This was not well recognized by linguistics researchers until late in history.

Many arose spontaneously as creoles from a pidgin sign language parent, the same way that spoken CreoleLanguage''''''s do.

(This is a stub. Sign Language is a vast subject with a long largely hidden history (http://www.milan1880.com/))

Other bits to mention would be...
* MakatonLanguage, a ConLang created by hearing people to teach communication with people who have communication and learning disabilities. 
* Fingerspelling and vocabulary. See: http://asl.ms
* Sign Artforms.
* Deaf culture.
* Separatism.
* Oralism.
-----
External links:
*[http://www.lifeprint.com/ ASL Resource Site] Free online sign language lessons, ASL dictionary, and lots of resources.

-----
CategoryNaturalLanguage