If you're looking for work right now (and, statistically speaking, there's a reasonable chance you are), part of your job search process should be finding out as much as you can about possible employers or clients. And what better way to do that than to hear from the people who have actually worked there?
Two sites that let you do just that are Glassdoor and Jobinions. Both of these sites let you read anonymous reviews, both positive and negative, of companies from past and current employees. Glassdoor also includes salary information for various positions (real numbers for specific positions and companies, too, not the abstract ranges you'll find at some job sites for general titles across an industry).
Of the two, Glassdoor has much more information, as Jobinions was just started this week and thus far only has two reviews. That said, Jobinions has big plans, including eventually creating reports on companies that give a summary of company characteristics based on the reviews submitted.
Of course, these sites will get better the more submissions they receive, so you should feel free to contribute (in fact, Glassdoor lets you see a lot more information if you do so). If you do that, be sure to follow some basic guidelines:
1. Post anonymously. Should be obvious, but still worth pointing out.
2. Be civil, fair, and mature. Criticism is fine -- that's why these sites exist, after all, not just for the benefit of prospective employees, but also so that employers can identify issues in their organizations and compare them to others. However, anonymous or not, you'll just look silly if you post critiques that are petty or immature.
3. Share the positive as well as the negative. If you really don't have anything positive to say, then fine, but it's good for companies to get credit where it's due. Plus, different people like to work for different types of organizations, and a place that other people hate may be just your thing.
I'm sure there are other sites out there like this as well. If you know of any, feel free to share them in the comments.