What version of the module to target? 2.x is fairly stable, with work underway on a 3.x branch.
Which version of Drupal to target? 7.x almost ready, 6.x well-established.
What is a REST API?
explanation
benefits
allows people, including you, to build applications on top of your website's data
costs
Services Module
what it does
how it works
authenticating
API Keys
OAuth
etc.
REST Server module
GET vs. POST
Write a custom parser
Build a resource module to handle GET requests with parameters coming through a URL