{ Full disclosure, I'm the author of WCFStorm.Rest }
If you're on Windows and is looking for an alternative, check out wcfstorm.rest (http://www.wcfstorm.com/wcf/learn-more-rest.aspx). It has a lot features similar to postman and adds some more, like saving requests and responses into functional test cases which includes a showing a graphical "diff" between the actual and expected responses as well as being to define custom validation rules that is executed against the http response. It can also do a single load test as well a distributed load test using several machines (http://www.wcfstorm.com/wcf/how-to--distributed-performance-...)
WcfStorm.Rest is a paid software but it has a LITE version which is available for free and works well for ad-hoc testing and exploring REST API endpoints.
If you're on Windows and is looking for an alternative, check out wcfstorm.rest (http://www.wcfstorm.com/wcf/learn-more-rest.aspx). It has a lot features similar to postman and adds some more, like saving requests and responses into functional test cases which includes a showing a graphical "diff" between the actual and expected responses as well as being to define custom validation rules that is executed against the http response. It can also do a single load test as well a distributed load test using several machines (http://www.wcfstorm.com/wcf/how-to--distributed-performance-...)
WcfStorm.Rest is a paid software but it has a LITE version which is available for free and works well for ad-hoc testing and exploring REST API endpoints.