VisBug 0.3.0 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The application lies within Developer Tools, more precisely Webmaster Tools. The actual developer of this free Mac application is Adam Argyle. We recommend checking the downloaded files with any free antivirus.
From the developer:
VisBug gives power to designers and content creators by bringing design tool interactions and hotkeys to the browser. With this app, you can edit or style any page, in any state, like it is an artboard. You can easily inspect styles, spacing, distance, accessibility, and alignment. The tool allows you to edit text and replace images.
You may want to check out more software for Mac, such as Url Extractor X, WebPics or JustinmindPrototyper, which might be similar to VisBug.