Reference:
All Music Guide (AMG)
Discogs

News/Magazines:
NME
Pitchfork
Spin

Free music:
Internet Archive

If you'd like to see something added, please send Mr.Samsa, Jain or Fallible a PM.

ClassicsToday
'CD reviews and other features.'

GMG Classical Music Forum
'A good discussion board.'

Gramophone
'The website of the venerable British magazine includes an archive of every single past issue, dating back to 1923.'

Courtesy of Shrunk.

Classical Convert

'Nice site for those that would like to give classical music a try, but have no idea where to start. Not perfect by any means, but a fun way to find out what one might like.'

Courtesy of Wilde.

Pitchfork

Daily news updates, reviews of new album releases across all genres of popular music, a great archive of interviews and articles, pitchfork.tv for videos and band profiles, and free mp3 songs from featured artists. Particularly good for indie, alternative, and electronica but honest and trustworthy criticism of all, whether major artists or up-and-coming bands.

All About Jazz

A one stop shop for everything to do with Jazz. An international site which nicely keeps apace with the work of contemporary innovators, and the plethora or reissues. News, reviews, articles, interviews, forums, guest contributors, playlists and free downloads, photo archive, podcasts, jazzvideoguy (youtube), musician and club database, guides, blogs, internet radio stations, store, small ads and personals, and more...

Absolute Punk

Not strictly Punk, but mostly. Non-partisan, international and all inclusive celebration of guitar driven contemporary music. The site also covers the Alternative and Indie end of the spectrum and includes all the usuals; News, reviews, artist profiles, blogs, gig guide, forums, gallery, interviews, new releases, articles, guest and member contributions, mp3's downloads.

Planet DnB

The internet hub for all things Jungle, DnB, and Dubstep. Yep, it has the news and reviews, internet radio, interviews, youtube-channel, and stuff, but the site is very much run for the benefit of a very active community with club and dj databases, chatrooms, forums, member contributions, gig, event and club guides, etc,. A great portal on to the growing DnB scene.

Courtesy of THWOTH.

last.fm

The description from Wikipedia:

Using a music recommender system called "Audioscrobbler", Last.fm builds a detailed profile of each user's musical taste by recording details of the songs the user listens to, either from Internet radio stations, or the user's computer or many portable music devices. This information is transferred to Last.fm's database ("scrobbled") either via the music player itself (Spotify, Amarok) or via a plugin installed into the user's music player. The profile data is then displayed on the user's profile page. The site offers numerous social networking features and can recommend and play artists similar to the user's favourites.

Courtesy of Brian.