My thesaurus is one of my most-used reference books. It's a massive book with a usable index and my own alphabetical labels. I've had it since high school graduation, and it continues to serve me well.
One of my most given pieces of advice for writers who want to improve is to use a thesaurus. Thesauruses can brighten dull writing, reduce redundancy, and allow the writer to more accurately express a thought or idea. They also increase vocabulary, synonym recognition, and use of creative expressions. If you've never used a thesaurus, shame on you! If you've been using one for years, congratulations. Even the best of writers sometimes stumble on a word or turn of phrase and need to have their vocabularies jogged when they aren't offering the appropriate words.
While I'm not particularly fond of the reference.com set of sites, Thesaurus.com is probably the only really useful web-based thesaurus. It pulls lots of results quickly for the word in question and doesn't require you to leave your keyboard to flip through a book. I still recommend getting a gigantic paper-based thesaurus, however, if you really want to do some serious writing.
My advice about using a thesaurus comes with a caveat: If you think the synonym you've identified is what you want, but you aren't absolutely sure, look it up. There are very few true synonyms, as each word is imbued with its own nuanced meaning. Always choose the best word for the job.