Ninjawords is an Ajax online Dictionary. Its very easy to use, enter a word and hit LookUp, it gives you the definition of the words. A nice feature is that you can even compare words by typing all at once in the search box separated by commas.
It pulls the definitions of the words from the free and open Princeton WordNet and wiktionary.
ObjectGraph is another online Web Dictionary implemented in Ajax. It works like Google Suggest. It suggests words as you type. It has many searching options like Classical, Elements, Thesarus.
I love these two and its really handy for me.