Pow, the latest project to come out of Ruby on Rails creators 37signals, is a server for developing Rails & Rack applications on OS X that requires no configuration and can be set up and going in less than a minute.
Other products such as Phusion Passenger have existed to make developing multiple Rails apps simultaneously easier, but they require a fair bit of configuration before you can get started on real work.
Pow was created by 37signals to get rid of the hassle that setting up a Rails development environment for every new developer or new computer in the company had become. Now, they say, the process only takes a few minutes.
Apps running under Pow use a hostname based on the symlink you choose to set them up with and the extension .dev, a fairly standard convention on local development servers.
Pow is an open source MIT licensed project. Check out 37signals’ screencast to see just how little is involved in setting Pow up.