require 'rubygems'
require 'sinatra'
get '/hi' do
  "Hello World!"
end

I can do four lines better than that, though:

require 'sinatra'

So what does that get you?

batkin:cholla ben$ ruby -rubygems cholla.rb
== Sinatra/0.9.0.4 has taken the stage on 4567 for development with backup from Thin
>> Thin web server (v1.0.0 codename That's What She Said)
>> Maximum connections set to 1024
>> Listening on 0.0.0.0:4567, CTRL+C to stop
127.0.0.1 - - [03/May/2009 10:13:46] "GET /index.html HTTP/1.1" 200 7 0.0019

A basic http file server, perfect for working with plain html, javascript, and CSS! Just create a public/ directory alongside the one-line sinatra app, and have at it! If you create an index.html file in public/, it will be served up when you go to http://localhost:4567/. Try it!

Once you decide to add some server-side code, you can simply go in to the sinatra app and add it.