Ninja Words is a nice online dictionary service. As they advertise on the site, it's very quick ... well, until another million surfers discover it, anyway.
Bucking the normal trend, it understands correct English spelling, rather than requiring the septic version.
It also accepts US spelling, which is sad, but fair enough, I guess ... they need all the help they can get :-).
Strangely, there doesn't seem to be a one-to-one correspondence between the provided definitions. For example, "color" includes usage as an adjective, but "colour" does not. Quite peculiar.