Downloading Umwelt 0.8.1 for Mac from the developer's website was possible when we last checked. We cannot confirm if there is a free download of this app available. The application lies within System Tools, more precisely Device Assistants. This free Mac application is a product of Gadgetish. We recommend checking the downloaded files with any free antivirus.
From the developer:
Umwelt locks your computer when you are away from it and unlocks it when you come back, automatically. By measuring the strength of the Bluetooth signal from your smartwatch, it "senses" when you are close or away. You can adjust the locking/unlocking distance very easily from the main menu. The Mac version works with any Bluetooth LE wearable device that you can pair with the computer.
You may want to check out more software for Mac, such as iStar Composer, toDoo or Matchprint Virtual, which might be similar to Umwelt.