I mentioned in the last post that I use WordWeb for my thesaurus. I like it for its simplicity and for the ability to access other sources. I have found over the years that there are good resources and there are not so good ones.
Some may not like Wikipedia due to its open edit vulnerability. And I say if you are going to depend on one resource for your research, then you are doing it wrong. Wikipedia is a vast resource and the ideal place to start. Their links at the bottom (see also, references, further reading, and/or external links), are the next step. Wikipedia is just one part of a wonderful organization called the Wikimedia Foundation. From images to quotes to even a species directory, Wikimedia Foundation provides a vast resource of information to get lost in.