WriteExpress Online Rhyming Dictionary

Online dictionary for poetry and songwriting.
http://www.WriteExpress.com/online.html

Semantic Rhyming Dictionary

Rhyming dictionary and thesaurus: rhymes, near rhymes, homophones synonyms, and semantic siblings of English words.
http://www.rhymezone.com/

Word Central

Student dictionary and rhyming dictionary online at Merriam-Webster's website. Daily Buzzword by subscription.
http://www.wordcentral.com/

AnalogX - Rhyme

[Windows] A rhyming program. Knows over 100,000 words. It can rhyme words forward, backwards, and with matching syllable counts.
http://www.analogx.com/contents/download/audio/rhyme.htm

Rhymesaurus

[Windows 95, 98, ME, NT and 2000] Software. Free trial version available for download. Rhyming dictionary and other lexical tools.
http://www.purpleroom.com/software/

Lexical FreeNet

Allows you to search for rhymes and relationships between words and concepts.
http://www.lexfn.com/

AnsMe.com Rhyming Dictionary

Enter a word in the search box and see a collection of words that rhyme with it.
http://rhymes.ansme.com/

Alcor Rhyming Dictionaries

Online rhyming dictionaries in Spanish, French, Italian, Portuguese, Romanian, and German.
http://www.alcor.com.au/rhyming_dictionary.asp