How can I implement a RESTful API in Drupal? Can any one please let me know what modules are available to implement a REST API? 

So far I've only found the Redmine REST API module, which is used for Redmine.
For a REST API implementation on Drupal, use the Services module.


  A standardized solution of integrating external applications with Drupal. Service callbacks may be used with multiple interfaces like REST, XMLRPC, JSON, JSON-RPC, SOAP, AMF, etc. This allows a Drupal site to provide web services via multiple interfaces while using the same callback code.