You could stop the internet from expanding for a little while. Buy out all the remaining IP addresses. There are only 4 trillion possibilities. Many are already taken...and some are reserved for other uses. Well, even if you had the money and the means...it would just push forward IPv6...so go for it
By just taking out a small chunk of the internet for a few seconds can cost thousands of dollars. It's kind of scary that anybody could really harness that power. To make a brick and mortar store lose money, you either need to rob it or shut it down. With ecommerce, you only need a simple little program. Anybody that knows how to run simple unix commands could take down the biggest website. I sure could. Am I stupid enough to? No.
DDoS floods are a pain. They're hard to trace (since they make use of insecure machines), they're hard to stop, and they can cost businesses lots of money. I remember a few months ago DALnet (a large IRC network) was being taken down by a bunch of kiddies. The network owners announced a massive price spike in their fees due to all the traffic the DOS'ers were sending through.
It's sad that there are people like this. It's sad that those people can have such a power. I imagine that the internet will severly get cut off some day. 'Hacking' (and I use that term very loosely...since it should rather be called 'cracking') has become way too simple. People don't bother to secure their machines or even bother to check the traffic going through them. It's also a bit of the software developers fault. To point out some popular ones...Microsoft knew about many of the security holes that worms and viruses use to work their way into machines. It was only patched _after_ the virus was released. Then there were still machines that didn't even apply the patch! It's a vicious circle. (I know that I still get some Codered hits on my website...so it is still going around.)
The _BIGGEST_ reason sites like Amazon and Yahoo can get taken down is due to the ignorance of some folks! Yes, it _is_ the fault of the person who initiates the DDoS, but if there were no machines to distribute the DoS attack to, there would be no problem. People have misconfigured Cisco routers running on high-bandwidth connections and they're getting badly abused.
Simple descriptions of DoS:
You ping one computer, they send back a response.
You ping a misconfigured computer, they send back multiple responses.
You ping a misconfigured computer with IP spoofing, they send back multiple responses to the spoofed IP. Therefore, somebody is getting packets they didn't ask for.
Send hundreds and thousands of those...so that it overwhelms the spoofed IP with packets, they go offline. That is a DoS.