This short list presents representative sites for use by clinicians, librarians, researchers and is not meant to be all-encompassing. They are reliable, fast-loading and free. Indices can be comprehensive and try to be inclusive. Medicine - Links to medicine and 9 other major subjects are available from this site. Medical Matrix - Large, comprehensive and worth a bookmark.
Registration is required, but it is free.
MedNETS - This site is a relative newcomer. There are sections for patient education, CME, journals and more. Registration is required in order to log on, but it is free. Webplaces - My favorite directory. Lists all the major individual search engines and indexes worldwide.