Personally, I prefer amateur sites when trying to discover new music, as they are run by one person and thus you can see how their tastes compare to yours, and can use that to see if you would like an album or not. Plus, a person doing this for fun can't really have an agenda. Plus, they generally like prog rock a lot more than mainstream publications do. Note: despite the fact that there are many, many great sites out there that are no longer being updated, I am only linking to currently updated sites, as anything else usually requires sifting through various layers of the Wayback Machine, which I'm too lazy to do right now. I might come back and add a couple of my favorite archived sites later, though.
Only Solitaire: George Starostin's Music Reviews: http://starling.rinet.ru/music/index.htm (note: link is to the old site, the blog can be accessed from the front page). One of the most comprehensive reviewers on the Web, and the definitive source for '60s music, George Starostin began his site seemingly in the late 90s, and updated into the mid 2000s. As well as being the source of my highlighting system for the track listings, George has one of the most comprehensive and best writing styles, and the man has reviewed just about anything up through the 70s, after which it is a bit more sporadic. Although he ceased updates of the original site in 2007, in late 2009 he began reviewing again, independent of his old reviews, on the Only Solitaire Blog, and I'm pleased to see that he no longer rabidly hates "Dream On." I read his stuff a lot more than I initially expected, considering the first words I read from the guy were" I HATE PINK FLOYD. "
Mark Prindle's Record Reviews: http://www.markprindle.com The original (from what I know of this, anyway), and one of the most popular web reviewers, Mark Prindle is both a great music critic and a hilarious writer. Even the reviews for bands you have no interest in are worth reading, as they are often both hilarious and informative (although people who are extremely easily offended should probably avoid doing this). Mark also frequently reviews bootlegs and compilations from the artists he reviews, so this site is probably the best resource for people looking for information on such things. Additionally, Mark Prindle is the most famous person to ever comment on my Facebook status.
John McFerrin's Rock and Prog Reviews: http://johnmcferrinmusicreviews.org/ The definitive source for progressive rock (a mantle I'd love to steal someday but doubt I will), John McFerrin may not have as large a count of artists as other sites, and he may only write four reviews a month, but his reviews are extremely comprehensive, well-written, and helpful. He is also extremely knowledgeable about the artists he reviews--in fact, much of the trivia I know about some of my favorite bands I originally learned from reading John's reviews.
Scott's Rock and Soul Album Reviews: http://www.sfloman.com/ Scott Floman is another excellent writer, who seems to be refreshingly positive about the music he reviews. Updated sporadically, but what's there is great. Much of his writings in the "lists" section of the web site are spot on, as well.