This page is almost impossible to find unless you know where it is. So here it is for the record.

http://developer.apple.com/ue/aqua/icons.html

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.