I'd use a LaunchAgent and WatchFolder. Haven't tested this, but it should work....
But to test, put the file below in /Library/LaunchAgents/com.company.mygreatTool.plist and a script to to whatever in place of script.sh below. Every time the download folder gets touched, the script will run.
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
<plist version="1.0">
<dict>
<key>Label</key>
<string>com.company.mygreatTool</string>
<key>EnableGlobbing</key>
<true/>
<key>ProgramArguments</key>
<array>
<string>/Library/path/to/script.sh</string>
</array>
<key>WatchPaths</key>
<array>
<string>~/Downloads/</string>
</array>
<key>OnDemand</key>
<true/>
</dict>
</plist>