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.
1<?xml version="1.0" encoding="UTF-8"?>
2<!DOCTYPE plist PUBLIC "-//Apple Computer//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
3<plist version="1.0">
4<dict>
5<key>Label</key>
6<string>com.company.mygreatTool</string>
7<key>EnableGlobbing</key>
8<true/>
9<key>ProgramArguments</key>
10<array>
11<string>/Library/path/to/script.sh</string>
12</array>
13<key>WatchPaths</key>
14<array>
15<string>~/Downloads/</string>
16</array>
17<key>OnDemand</key>
18<true/>
19</dict>
20</plist>