DiskSpy for Mac

From Nick Zitzmann:

While many PC makers put hard disk LEDs on the outside of their computers, Apple hasn't put any disk LEDs on any Macintoshes since the SE/30. Some Mac old-timers may remember the number of products that were available during the System 7 era that provided a software disk LED in the menu bar.

DiskSpy is a small program that acts as a software disk LED for Macs running Mac OS X. DiskSpy sits in the menu bar and gives visual notifications when the OS is reading from or writing to a disk. DiskSpy does not run in the kernel region, does not use any private frameworks or undocumented APIs, does not require any hacks to use, does not trap any low-level I/O calls, does not run as the super user, nor does it actually write to any disk(s) except to save its preferences, so it is safe to use. And the icons can be customized.