Download of Multimode 3.9 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 application lies within Education Tools, more precisely Science. We recommend checking the downloaded files with any free antivirus.
From the developer:
MultiMode is a shareware application which allows your Mac to decode and transmit several different modes of non-voice communication often heard on shortwave and ham radio. It is the only program of its kind for the Mac, rivaling commercial packages for the PC platform costing hundreds of dollars. It is ideal for amateur radio operators, shortwave radio listeners, and other radio enthusiasts.
You may want to check out more Mac applications, such as MultiMode Cocoa, Ultra Analog or AudioDesk™, which might be related to Multimode.