AudioLocator
http://www.audiolocator.com/
AudioLocator searches for mp3's on ftp servers

Planet.MP3Find    http://www.innovators.com/products/PlanetGroup/planetmp3find.asp    Planet.MP3Find is an advanced ftp search tool with an integrated ftp client

Findsongs/Palavista
http://www.findsongs.com/
A MetaCrawling web-based search that looks to ftp servers for it's results

SearchLord
http://www.searchlord.com/
Searching the visible and invisible web via multiple vertical metasearch engines, specialty searches and web category subjects.