Also, although commercial directories continue to place a smaller role, THIS DOES not mean that several general and many vertical directories (most non-commercial) are out there. Most of these resources strive for quality vs. quantity, have collection development policies (what they will or will not add), and some even offer RSS feeds and/or “what’s new” newsletters.

+ LII.org
http://www.lii.org
Well organized, excellent annotations

+ Intute
http://intute.ac.uk
What’s New
http://www.intute.ac.uk/latest.html

+ Infomine
http://infomine.ucr.edu

+ IPL
http://www.ipl.org

+ AcademicInfo
http://www.academicinfo.net/table.html
The work of Mike Madin

Verticals (A Few Examples)

+ Business
http://globaledge.msu.edu/ibrd/

Education, Classroom Materials
+ The Gateway
http://www.thegateway.org/browse
and
FREE
http://www.free.ed.gov

+ Science
National Science Digital Library
http://nsdl.org/browse/

+ Museums
http://www.musee-online.org/
http://www.museumland.com/

+ Medicine/Health
MedlinePlus
http://www.medlineplus.gov
From the National Library of Medicine
Including thousands of pages for diseases and conditions.
http://www.nlm.nih.gov/medlineplus/healthtopics.html
Drugs, Supplements, and Herbal Information
http://www.nlm.nih.gov/medlineplus/druginformation.html
Tutorials
http://www.nlm.nih.gov/medlineplus/tutorial.html