The 1.2.5 version of SIDT for Mac was available to download on the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The actual developer of this free Mac application is XU Zhao. We recommend checking the downloaded files with any free antivirus. SIDT for Mac lies within Developer Tools, more precisely Webmaster Tools.
From the developer:
SIDT is a developer HTTP request tool designed to help you debug your request and API. The application provides handy request parameters and HTTP header editors, Base64 encoding, and timeout settings. The response Data can be displayed in Web/Text/JSON format mode.
You may want to check out more software for Mac, such as Multisite for iWeb, HyperEdit or Validator-SAC, which might be related to SIDT.