This page is almost impossible to find unless you know where it is. So here it is for the record.
Includes an Aqua Icon design kit, links to icon designers and design tools.
20 January 2005: This is no longer there. The AquaIconKit is no longer available, but Apple has said they plan to update and re-release it.
However, the AquaIconKit is available unofficially from mirrors on the AquaIconKit page.
So along with those, have some RoyaltyFreeIcons.