3.1. Installing Mongrel
Installing Mongrel is a snap. The only prerequisites are Ruby 1.8.4 or greater and RubyGems. If you don't have these already, you'll have a hard time running Ruby at all. For help with installing these on your system, please refer to the Ruby Web site (http://www.ruby-lang.org/en/), but in most cases you will want to install it.
You can install Mongrel in one step from your command line:
$ sudo gem install mongrel --include-dependencies
This will install the Mongrel gem and its dependencies, which are gem_plugin and, depending on your platform, either daemons or win32-service. If you're not on Windows you'll see Mongrel build the C extension during the gem install. Check the mailing list and Web site if you run into any issues, as it's probably caused by a funky Ruby installation.
3.1.1. Mongrel Pre-Releases
If you are the risk-taking type (you are using Ruby after all, aren't you?), then you can install and help test Pre-Release gems from Mongrel's Pre-Release repository. Like most pre-release code, these gems typically include new features and/or bugfixes that need some platform testing, and overall they're pretty stable. Ruby tends to introduce some interesting system-level differences, so testing on different platforms is extra important.
$ sudo gem install mongrel --source mongrel.rubyforge.org/releases