One of the most valuable online writing tools I've found is The Visual Thesarus. I think I read about it in a writer's magazine, otherwise I'm not sure how I discovered it, but it's fabulous for those frozen moments when the right word won't rise from the subconscious.
Allow popups and make sure your Java runtime environment is enabled then visit the website for a trial. When a word is typed in, almost immediately if you have a fast connection, a tree of alternate words is provided and definitions are available with a click.
Point to a new word and another word tree is produced introducing you to an even broader vocabulary. As with any thesarus, some words are archaic, but sometimes those are fabulous tidbits of knowledged.
I never knew a susurration described the "indistinct sound of whispering."
If it won't cure writer's block, it will entertain wordsmiths while waiting for blockages to clear.