Freeware app allows you to monitor all file system changes; uses the same underlying mechanism for noticing file system changes as Spotlight. (Via Rentzsch.)
Update: Siracusa has informed me that the fsevents API is undocumented and not supported for third-party use, and that apps that play with it like fseventer risk hosing Spotlight. So consider yourself warned.
★ Monday, 30 January 2006