Reallysqs is a project mainly written in Java, it's free.
A VERY simple implementation of the Amazon SQS Api
This project has been created to provide a very simple, local implementation of the Amazon Simple Queue Service, as specified here:
http://docs.amazonwebservices.com/AWSSimpleQueueService/2006-04-01/REST_RestOperations.html
The project is broken into several implementations so that comparisons and benchmarks can be carried out across various frameworks/languages.