Computing in the Cloud with EC2.
When I first started learning about EC2, I wasn't so sure, but I'm a total convert now, it's just so beautiful.
EC2 is Amazon's new service, still in beta, that lets you deploy pay-as-you-go servers. You pay 10 cents per hour, which works out to about $70 a month. For this you get the equivalent of a 1.7GHz Xeon with 1.75GB RAM and a 160MB hard disk. This is pretty close to equivalent with other dedicated servers that you can get.
However, what makes EC2 so sweet is that this is all virtual and pay-as-you-go, so you could run one server for a month for $70, or run 700 servers for an hour for $70. It's really hard to decide how many servers to get for a website, and with most current systems, it's quite a chore to get your new computers online and configured. With EC2, it's just a simple command, and your new instance is deployed.
We're using EC2 for the startup that I'm currently involved in, we should be launching publically very soon, I'll give you more details then.