DNS/Proxy based DDoS protection

Hi, Over the last several years I've noticed there seems to be no limit to the number of proxy/DNS based DDoS protection services springing up all over the place so I am wondering if anyone has any insights on what sorts of tools, etc these companies use to provide this service (Open Source, Commercial?) and if anyone has any thoughts or experiences regarding the effectiveness of these types of services to protect HTTP services. thanks, -Drew