OS X believes that the Application bundle is signed, but the contents have been tampered with.
If I get some time later, I'll play with [re]signing it myself with a self-issued (and trusted) certificate.
~/Downloads>: openssl md5 FreeRapid-0.9.dmg
MD5(FreeRapid-0.9.dmg)= 3e4a14c47ee2cc2b63329165622edf33
~/Downloads>: hdiutil attach FreeRapid-0.9.dmg
/dev/disk3 Apple_partition_scheme
/dev/disk3s1 Apple_partition_map
/dev/disk3s2 Apple_HFS /Volumes/FreeRapid-0.9
~/Downloads>: codesign -vvvv /Volumes/FreeRapid-0.9/FreeRapid.app/
/Volumes/FreeRapid-0.9/FreeRapid.app/: invalid signature (code or signature have been modified)
In architecture: i386