Here are some of the sites I use on a daily basis to help inform me regarding whether I want to buy a stock or not and if so - for how long.
ClearStation: Tag&Bag - I use this site to find candidates for day and swing trading. It has a slew of pre-formatted screens and I especially like the gap up and gap down screens for finding - what else - gap ups and gap downs. But on a Sunday afternoon or evening when there isn't anything else going on that might interest you there are also screens such as - weeks best trend and most active and breakouts and so on that you might use to garner a list of stocks for further investigation. The part I like best is the fact that they give the price and the volume on the screen so that you are not wasting your time looking at stocks that might not fit your basic parameters.
US Earnings - This site provided by Yahoo Finance keeps me up to date on when my various portfolio holdings are coming up for earnings. It is up to me to decide whether I want to play earnings roulette or not.
WhistperNumber - Sometimes a stock has some indication of a lower or higher than expected earnings number. This site attempts to keep all of that information in one place.
Ivolatility - I use this one to give me an idea of both historical and implied volatility of a stock that I either own or am considering buying. Readers know that I am big on volatility and this site is one of the best when I need a quick answer as to whether the market thinks the stock is going to move or not. Implied volatility is both very important and very complex and if you don't know much about it I suggest you do some discovery work. As always you can start here on Wikipedia.
And of course you all know that I use stockfetcher.com, prophet.net and quotetracker.com to support my continuing trading activities.
I don't get anything from any of the companies mentioned.