JIRA's got a real REST API now:
REST easy with JIRA 5 | Atlassian Blogs: Now that JIRA 5 is out, let’s talk about one of my favorite features of this new release, JIRA’s new REST API. JIRA has supported remote APIs for many years with SOAP, XML-RPC, and JSON-RPC. However, telling developers that you support SOAP (and only SOAP) is like saying that you like writing applications with COBOL — it’s out of style. Today’s cool kids gravitate towards REST. It’s clean, simple, and highly portable across languages and frameworks.
An alternative to Atlassian's new API is the recently release Rational OSLC Adapter for JIRA, which allows you to do more sophisticated integrations with JIRA including delegated UIs for issue creation and selection.
It's kind of surprising to me that JIRA does not have a "REST API." Looking on the bright side, this may be an opportunity for Open Services for Lifecycle Collaboration (OSLC) to show its value, so back in March I made a little pitch in the appropriate JIRA issue:
Dave Johnson added a comment - 10/Mar/10 9:24 AM - edited
You should consider making the JIRA REST API conformant with the Open Services for Lifecycle Collaboration (OSLC) interface for Change Management (OSLC-CM). This would allow JIRA to integrate with ALM tools from IBM, Oracle, Rally, SourceGear, etc. The Mylyn folks are already involved. Here's the link to the OSLC-CM home page: http://open-services.net/bin/view/Main/CmHome. OSLC-CM v1 is the current spec and work on v2 is underway. It's an open effort and we'd love JIRA and/or JIRA users to join in and help us define v2.