Also highly recommended is the Earth and Sky programme's Skywatching Center. This very attractive and enjoyable educational site features illustrated nightly reports which regularly focus on the Moon.
John Walker discusses the varying appearance of Moon in depth on his Inconstant Moon page. Although it shares its title with this site, there is no plagiarism involved - whilst I have borrowed from Shakespeare, John has drawn on a short story by Larry Niven!
For general information and some useful links try the National Space Science Data Center's page The Moon, Carl Koppeschaar's Moon Handbook or Gordy Braun's Lunarama.
Finally, Niels Zagers shows how the appearance of The Moon changes when viewed with the naked eye, binoculars, telescope and even a visit!