I just made a preview release of Instant Rails:
Instant Rails is a one-stop Rails runtime solution containing Ruby, Rails, Apache, and MySQL, all preconfigured and ready to run. No installer, you simply drop it into the directory of your choice and run it. It does not modify your system environment. See the Release Notes for a complete list of what is included.
Instant Rails is still under development and currently has only a preview release. There are still a number of rough edges, but it is essentially functional and usable. This initial release of Instant Rails is for Windows, but there are plans for ports to Linux, BSD, and OSX.