https://en.bitcoin.it/wiki/Trade (Almost every service which deals with Bitcoin is listed here)
http://bitcoin.clarkmoody.com/ (Check the price of BTC with an updated "last" price in the website header, so no need to open a tab when using a multi-tab browser like Chrome or Firefox -- now reconnects if disconnected!)
https://glbse.com (Buy/sell BTC-denominated stocks)
https://bitcoinica.com/ (The bane of Bitcoin. Leverage yourself 10:1 and take a gamble on your projection of the Bitcoin price!)
Oh, and that. (lots of Bitcoin discussion & news)
ETA:
http://bittalk.tv/?page_id=314 (Bitcoin Magazine -- get the BTC goings-on in a well-organized manner on physical media!)
ETA2: http://<i>Linking to illegal sites is forbidden. If you bypass this censorship, you will be banned.</i>.onion/ ETA3: uh... guess you can skip that one
ETA4:
http://blockchain.info/ (Having issues with a transaction? Check out what happened here),
http://bitcoincharts.com/ (charts with plenty of options for how to view the data -- also shows arbitrage opportunities between exchanges)
(8 sites, now, 9 depending on your criteria for what counts)
All else is irrelevant.