This is just my own personal archive of online tools that I employ on a regular basis when crafting my next lyrical monstrosity.
Thesaurus – Words that are similar, synonyms, antonyms
http://www.merriam-webster.com/ – This one has annoying pop-ups and is smoking crack if they think anyone will pay for a subscription.
Words that rhyme
http://www.b-rhymes.com – An oldie but a goodie, too bad they limit the results to 100.
http://www.rhymezone.com – This site is awesome, you can break it down by syllables, and the words that are “near rhymes” feature is the bomb.
Words that start/end with a letter or search pattern