I’m not sure if it is the first signed third-party application to take advantage of Mac OS X Leopard’s new code signing feature. However, it’s at least the first one I’ve seen publicly.
The release notes for Fetch 5.3 show a raft of updates for the little niceties of Leopard. Put Automator actions under the “Internet” category? Check. Prevent Time Machine backups of Fetch’s cache? Check. Support the new application level firewall (ALF)? Check. Make the appearance match Leopard’s unified style? Check. And so on.