You probably already know the define: operator of Google for a long time, which serves as a quasi- online dictionary. However, do you realize that Google have a real dictionary service (which is, as you guessed it, free!)? Like the operator mentioned, the Google Dictionary provides you with definitions of a word. However, it is much more formatted than that of the define: operator, as well as enriched with example sentences; more importantly, it supports more than a dozen languages, and can help users translate around! You may also "star"/bookmark a word and review later. So besides dictionary.com and W-M (or any other such services you are accustomed to), you now have another choice.
P.S. I've enabled the Followers gadget of this blog. You can now openly (or secretly, if you wish) follow Hanrizon, if you have a Google account. The gadget is found at the bottom of the sidebar.