Click http://www.m-w.com/
and a link will take you to the Merriam-Webster Collegiate Dictionary and Thesaurus website. Entering a word in the top slot there gets you the dictionary and the middle slot gets the thesaurus. It's American, of course, but effective.
I've often found the thesaurus that comes with icrosoft word useful, but for an online one try www.bartleby.com, which is a collection of lots and lots of reference books. simply type in the word and select thesaurus from the drop down - i'm sure you can work it out!