Aside from using these (I am "TheSlayer" at Stocktwits), I have no affiliation with any of the below.
Finviz - its Screener is one of the best. Also try Yahoo's screener and Google's.
Twitter - if you are going to trade, use Twitter. And follow me: @The__Slayer.
Stocktwits - probably the best free chat on the Internet for stock traders, in Twitter form.
Yahoo Finance - no better place to do preliminary due diligence for your trades.
Stockcharts - despite having an elaborate chart package, I still use stockcharts from time to time
The Pattern Site - another charting site, mainly good for candlestick studies.
St. Louis Fed - best place to do research and plotting of government economic data. Priceless.
Bloomberg - good news site. Also try Reuters and Marketwatch.
Insider Transactions - not everybody thinks these are important, but Barron's has a good page.
Nasdaq.com - lots here, this link take you to the pre-market quotes page
Candlestick Trading Forum - Don't use the service, but has a good summary of Candlestick patterns
Major World Indices - Yahoo has a good page to see how European, Asian etc. markets are doing
CME Group Holiday Schedule - Find out when the markets will be trading
Tel Aviv Stock Market - This gives you something to think about during Sunday afternoons.
Euro Crisis Tracker - this page has come in handy occasionally, a Wall Street Journal summary.
Zerohedge - follow on Twitter @Zerohedge. Provides unique information - as long as it's negative.
Baltic Dry Index - This is useful in gauging overall economic health in Europe.
Slope of Hope - Possibly the most openly Bearish site, aside from Zerohedge, good traders there.
Shortsqueeze.com - Gives you changes in short interest in equities. More fun than useful.
The following I don't really use, but you might find them useful:
This is Money (UK)
The Street (Cramer's website)
The Motley Fool
Raging Bull - these message boards used to be big, now not so much