iMacMonitor 1.0 for Mac was available to download from the developer's website when we last checked. We cannot confirm if there is a free download of this app available.
Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. The software lies within System Tools, more precisely Device Assistants. The actual developer of this free software for Mac is Isam.
From the developer:
iMacMonitor is a small application running on the “display” iMac that monitors the Thunderbolt bus and automatically switches to Target Display Mode if necessary. Thereby one only has to connect the main computer to the Thunderbolt cable and a few seconds later the iMac can be used as a secondary monitor.
You may want to check out more Mac applications, such as ToFi, iStar Composer or Portable Nvu, which might be similar to iMacMonitor.