Clements' Guide to Information Architecture
A guide maintained by Maureen Clements that covers societies and associations, books, journals, how-to sites, government and education sites, consulting firms, and other resources related to information architecture.

InfoDesign
A fascinating portal to information design resources, with significant overlap into information architecture topics.

Info.Design (IA Resources)
An annotated list of information resources, maintained by Info. Design, an information architecture consultancy and think tank.

jjg.net
A brief guide maintained by Jesse James Garrett that positions information architecture as an element of user experience.

Search Tools
A section within the best guide to choosing and using search engines, SearchTools.com by Avi Rappoport.

Usable Web
A section within Keith Instone's Usable Web, named the "The ultimate comprehensive collection of useful links" by Jakob Nielsen.