Online Textbooks

Online Biology Book

Pubmed Bookshelf: searcheable online textbooks (Textbooks titles include Cell Biology, Biochemistry, Developmental Biology, Genetics, Immunology, Microbiology, Endocrinology, Neurobiology, Genomics, C++ Computer Programming, etc.

Wikimedia science bookshelf

Online Encyclopedias

Wikipedia

http://www.reverse-lookup.com/encyclop.htm

Encyclopedia of Sign Language

HyperHistory

Encyclopedia of Philosophy

Encyberpedia

Online Dictionaries

www.m-w.com

www.dictionary.com

Wiktionary

Wikipedia's list of dictionaries (including multilingual dictionaries)

http://www.reference.com/

http://www.reverse-lookup.com/dictionat.htm

Medical Dictionaries

http://dictionary.reference.com/others/Medical/

http://cancerweb.ncl.ac.uk/omd/

specialized dictionaries

http://dictionary.reference.com/others/

Online Thesauri

http://thesaurus.reference.com/

http://en.wikipedia.org/wiki/Thesaurus

Online Rhyming Dictionaries

Write Express Online Rhyming Dictionary: Online dictionary for poetry and songwriting.

Word Central: Student dictionary and rhyming dictionary online at Merriam-Webster's website. Daily Buzzword by subscription.

Semantic Rhyming Dictionary: Doubles as a rhyming dictionary and thesaurus. Find rhymes, near rhymes, homophones synonyms, and semantic siblings of any English word.