Penny stocks are highly prone to investing scams. Since penny stocks have a low market capitalization (price of stock times # of shares) they can be easily influenced by small amounts of investors.
One famous scheme is called the pump and dump. An example of this would be:
Investor buys a large amount of cheap penny stock (under a buck) over a long period of time (as to not effect the price). Then the investor sends out faxes and emails claiming the company he just bought stock in is the next Microsoft. After a bunch of gullible investors frantically buy the stock, it drives the price up. Once the buying is at its peak the investor starts selling. the investor walks away with a ton of cash while everyone else not in the loop loses.
This scheme is against the law and is enforced by the SEC.