For a small membership fee, you are allowed access to hundreds of legitimate jobs. Some claim to have researched the postings validity and claim they have more postings than any other site.
There are far too many free job boards to require anyone to pay for the service. Warning, many legitimate job boards are being targeted by criminals who then solicit work from home frauds. Although job boards such as Craig's List, Monster and CareerBuilders have systems in place to try and "sniff" out fraudulent posts and remove them; it is not a perfect system.
Be wary of any job offer you receive even if it comes from a legitimate board. Do not respond to any email from someone who does not list their full name, company name and address and a phone number. Also, use the internet to find the company's web site and verify this information. See if the job is listed on their career page. Do not give your social security number, credit card number, or bank account numbers or make any monetary transactions until you've verified all information. Also, don't give out any personal non-work related information, such as height, weight, or marital status.
Still not sure they are scams? Check out these links for more information. Note, these are not links to "legitimate" companies. These are links to articles by the BBB, FTC and new organizations exposing the frauds.