This tutorial will help you in understanding JSON and how to use it within various programming languages like PHP, PERL, Python, Ruby, Java etc.
1 - JSON Quick Reference Guide
A quick JSON reference guide for Programmers.
2 - JSON Useful Resources
A collection of JSON Sites, Books and Articles is given at this page.
3 - Java Tutorials
If you are starting with Java then it's best place to start with.
4 - JSON Tutorial in PDF
Download a quick JSON tutorial in PDF format.