Glassdoor is a site where you can get inside information on what it's really like to work at a company - from people who actually work there (or have worked there).
Glassdoor works like this: you can share an employer review or salary information, anonymously and for free. The community at Glassdoor then can see your content; again, your personal information is absolutely anonymous.
At the time of this writing, Glassdoor is limited to companies in the San Francisco area, but they will soon be expanding to other geographic areas. This looks to be an excellent resource for job seekers.