Ask any good career coach and they'll tell you that you can't do enough research on prospective employers.  And they'll also tell you, most people skip this step.

I know that the fact I was able to specifically name products in an interview, and asked the hiring manager several directed questions seeking out details on practices and metrics got me the third interview.  And that would not have been possible without research.

The absolute basics to researching a company are:

Check their website (about the product, about their media coverage, all of it)

Google the company, the product(s) and the people

Reference check the people on LinkedIn

Research them in the local business journal(s) or industry publications

Check professional association sites

You should have some basic company history and overview information, and try to get some background on the people you'll be interviewing with, as well as top people within the company (if you aren't interviewing with them).  Be able to talk intelligently about their business, and be able to demonstrate you're interested in what they do.

And don't think you've won a Get Out of Jail Free card if it's a startup.  Then you have to do more research on the individuals involved and their previous positions, dig up venture capital related details, and even surf field/industry/product related blogs. 

If you don't do this, you become a generic candidate, and that will quickly get you into the slush pile.  Think about it, who would you hire, the person who actually makes an effort, or the slacker?

You, the job seeker, have to be more of a detective than the HR/Hiring Manager team, because you have more variables to check.  You have more to lose as well.  Root out the deep darks they don't want you to know, try to find the red flags that may make them a less appealing prospect. 

And if you're really lucky, you'll find there aren't any seriously deep dark secrets, and you don't find the red flags, and they end up being an ideal match.