Download of SafariSingleWindow 1.1 for Mac was 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 software for Mac is Brodie Glen. The program lies within Internet & Network Tools, more precisely Browsers. We recommend checking the downloaded files with any free antivirus.
From the developer:
SafariSingleWindow is a SIMBL/PlugSuit plugin for Safari that forces all links that would open in new windows to open in new tabs instead. No configuration is necessary. You just open the .dmg file, run Install, and restart Safari. To uninstall, you simply run Uninstall from the .dmg.
You may want to check out more Mac applications, such as Portable Cyberduck, WebNative-Browser or WebCopier Mobile, which might be related to SafariSingleWindow.