Any client application that can send HTTPS requests can be an appropriate tool for developing RESTful applications with the VMware Cloud Director API.
REST client plug-ins are available for most browsers and many IDEs. Many of the examples in the VMware Cloud Director API Programming Guide vCloud Air Compute Service Programming Guide were developed using two open-source programs: cURL (http://curl.haxx.se/) and the RESTclient (http://code.google.com/p/rest-client/).
VMware provides additional SDK products that implement language-specific bindings for the VMware Cloud Director API, and include their own HTTP client capability. See http://communities.vmware.com/community/developer/forums.