Thesaurus recommendation?

As my Spanish vocabulary has increased I am beginning to learn different words to describe the same thing. When I use a different word than a lesson I often want to look them up to make sure they are synonyms. Does anyone have a recommendation for a good online thesaurus?

I have yet to use a tesauro en español (I had to look that word up...) - but it's a good idea. One web site that I came across is sinónimo.es ("http://www.xn--sinnimo-n0a.es/").