Company websites should be your first stop when researching for a job interview. These sites should give you an idea of what the company does and why they believe it's important, as well as their goals for the future. Look for "About" and "News" pages (or ones with similar names) to find behind the scenes information.
Also, for publicly-traded companies, make sure to review recent SEC filings, the company's annual report (or 10-K), presentations to stockholders and other corporate information usually found in the investors portion of the site. If you can't find the investors section, try googling: insertcompanyname investor relations to get there.
In addition to the company's website, check out the resources below. These databases are from the WVU Libraries and were deigned to help businesses investigate their competitors and the health of industries, which makes them great tools for you to use when preparing for interviews.