Here are a couple of interesting resources which may be useful for REX developers:

HAL specification
A good description for the hypertext application language
JSON validator
A nice online formatting tool for not-formated JSON strings
JSON to C# converter
An code generator for JSON structures into C# classes
Postman application
A desktop application which easily allows to send HTTP verbs to a server
A comand line tool for sending HTTP verbs
A REST client for sending REST requests, an alternative to Postman