A significant part of investing is getting solid information on companies before you purchase their stock. Below are the information sources that help you do your own due diligence in terms of understanding a company's strategy, future performance and potential value as an investment.
And don't dismiss the importance/value of Googling your company, especially for news stories. The best investing advice I ever received was from a finance professor when I was an undergraduate: The companies with the best stories to tell have the best potential as an investment. Mine Google for good stories. Better yet, set up a Google Alert to track news and information about exciting new innovative products and services (which are often the key to company's story.)
SEC Filings can be enormously useful for investors provided you use them efficiently. There are several that should be considered including the 10-K. Proxy Statement and 8-K.s
The 10-K is the annual report to shareholders. Sections most useful are the business description and risk factors (Items 1 and 1A). These give a great overview of strategy. Also worth reviewing is the Managements Discussion and Analysis of Financial Conditions and Results of Operations, also known as the MDA (Item 7), which gives a detailed overview of financial and operational performance.
8-K's are useful since they disclose material events that potentially impact the stock's value. They are irregular filings that are submitted in response to events (future or recent).
Finally, the Proxy Statement is an oftentimes overlooked treasure trove of information for investors. Investopedia provides a nice overview of how proxy statements can help investors.
Estatement Studies is a premium database that provides industry norms data. It also has a valuable tool for analyzing a company's financial position compared to its peers. You can access the client comparison spreadsheet when you download/export any report after selecting an industry.
Below are premium, fee-based information source for equities investing research to which the library subscribes. Standard and Poors can be particularly useful for your research since they have stock reports, an advanced stock screener, industry analysis and more.
Stock screeners can be useful in terms of helping you narrow purchase options. They are, however, only as good as the perosn who uses them in terms of investing decision-making strength. They require the user to have solid, researched criteria as well as good knowledge of the companies involved. none of which comes from the stock screener itself. Investopedia has a good primer on stock screeners and how they are used.