They're all scams. They tell you to stop paying your bills, send the money to them, and they'll get a settlement for a "fraction of what you owe". Most take your money and run, many charge outragous "fees", thus giving you less money to bargin with, and all will trash your credit and likely get you sent to collections and then eventually sued.
Credit counseling is a horse of a different color, and CCCS run by United Way is a good one. They negotiate with your creditors FIRST, and then once they get your payments and interest lowered, you pay CCCS and they pay your creditors, and they only charge a very smal fee to cover their overhead. Before doing CCCS I would first contact your creditors and see if they are willing to work with you first though. If they tell you to go fly a kite, call up CCCS for a consultation, and see what they can do for you (you don't have to accept if you don't like the numbers).