A few months ago Thesaurus.com started flagging areas of their website as "premium content" and they began looking for users to pay for services. As I am not a big fan of paying for things that used to be free, I went looking for another option.
In my travels I came across a fee-based subscription service called Visual Thesaurus by Thinkmap. Basically it is just a thesaurus, but the terms are clustered like a road map. This type of display makes it a whole lot easier to find the terms that match the definition that you are looking for. Visual Thesaurus has a few different plans, notable is a web-based plan costing $19.95/year.
While I am not entirely sure that I want to pay for an on-line Thesaurus, I have to give the Thinkmap folks credit for coming up with something that is pretty cool and useful.