Write Rhymes, www.writerhymes.com is a new site that can help you when lyric writing. Type a word, option-click it and a bubble pops up with words that rhyme. I have a bunch of these kinds of sites book marked including my favorite thesaurus.reference.com. Write Rhymes also has convinent save button which quickly shoots a .txt file of your text onto your desktop.
If your looking for a full featured song writing helper and database check out MasterWriter: www.masterwriter.com