Using anonymous sources is a valuable tool for a journalist, but it should be used sparingly. When a journalist quotes a source, the reader (or viewer) can judge the quote based upon how credible they think the source is (or the organization they represent). When a journalist quotes an anonymous source, he's essentially trading his on own credibility. The reader has to trust that the reporter is quoting a trustworthy source.
The next time you read an article where an anonymous source is quoted, ask yourself *why* the source should be anonymous. If it's an anonymous source in the Bush administration that's quoted bolstering the case for a war against Iran, then maybe you should wonder. Anonymous sources are especially useful in the context of a wider investigation, with corroborating evidence other than the word of the source. Ratbastid mentions Watergate, but remember that there was quite a lot of evidence in the Watergate affair. Deep Throat helped lead Woodward and Bernstein to a lot of important evidence - if it had just been his word with no evidence to back it up, Watergate wouldn't have been anything but gossip.
Glen Greenwald has been trashing the media for a while now about it's use of anonymous sources. For example:
http://www.salon.com/opinion/greenwa...4/05/abc_news/