Hello Everyone !!
I have released a captcha plugin Simple Captcha. It is really simple to implement, and provides a cool feature of multiple styles of images.

ruby script/plugin install http://svn.2750flesk.com/validates_captcha

Make sure that you can now see the directory vedor/plugins/validates_captcha.

Now run these commands from your application root to make the image and data directories

  ruby script/generate captcha store_directory
  ruby script/generate captcha image_directory

Here is the complete API for the usage of this plugin. I am describing the same idea as given in this API but in a bit more specific means.

Lets consider a model User in which we will implement the captcha.
Add the following code in the file app/models/user.rb

  class User < ActiveRecord::Base

    validates_captcha :if => :request_captcha_validation?
    attr_accessor :request_captcha_validation

    def request_captcha_validation?
      (self.request_captcha_validation==true)? true : false
    end

  end

Handle View and Controller

Add the code in the view inside your existing form.

  <% c = prepare_captcha :type => :image -%>
  <%= captcha_hidden_field c, 'user' %>
  <%= captcha_image_tag c %>
  <%= captcha_label 'user', 'Type in the text from the image above' %>
  <%= captcha_text_field 'user' %>

Your controller will look like

  def save
    # the line in bold represents that you need captcha validation.
    # if captcha validation is not required then remove this line from your controller.
    @user = User.new(params[:user])
    @user.request_captcha_validation = true
    @user.save
  end

However image is too noisy and it contains repeated strings.
To improve the quality of images generated by the plugin validates_captcha visit Here.

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require 'digest/sha1'
module AngryMidgetPluginsInc #:nodoc:
# CaptchaChallenge
class CaptchaChallenge
include CaptchaConfig
extend CaptchaConfig
DEFAULT_TTL = 1200#Lifetime in seconds. Default is 20 minutes.

attr_reader :id, :created_at
attr_accessor :ttl
  def initialize(options = {}) #:nodoc:
    generate_id
    options = {
    :ttl => config['default_ttl'] || DEFAULT_TTL}.update(options)
    self.ttl = options[:ttl]
    @created_at = Time.now
    self.class.prune
  end

# Implement in subclasses.
  def correct? #:nodoc:
    raise NotImplementedError
  end

private

  def generate_id #:nodoc:
    self.id = Digest::SHA1.hexdigest(Time.now.to_s+rand.to_s)
  end

  def id=(i) #:nodoc:
    @id = i
  end

  def write_to_store #:nodoc:
    store.transaction{
    store[:captchas] = Array.new unless store.root?(:captchas)
    store[:captchas] <<> c.created_at+c.ttl
    store[:captchas].delete_at(i)
  end
}
end
}
end#prune
end#class << words =" 'gorilla" default_dir =" 'captcha'#public/images/captcha" write_dir =" File.join(RAILS_ROOT," default_filetype =" 'jpg'" options =" {})" options =" {" string =""> config['words'] ? config['words'][rand(config['words'].size)] : WORDS[rand(WORDS.size)],
:dir => config['default_dir'] || DEFAULT_DIR,
:filetype => config['default_filetype'] || DEFAULT_FILETYPE
}.update(options)

#changed
self.string = Digest::SHA1.hexdigest(Time.now.to_s)[0..4] #options[:string]
self.dir = options[:dir]
self.filetype = options[:filetype]
self.filename = options[:filename] || generate_filename

write_to_store
end

# Generates the image.
def generate(options = {})
options = {
:fontsize => 50,
:padding => 20,
:color => '#000',
:background => '#fff',
:fontweight => 'bold',
:rotate => true
}.update(options)

options[:fontweight] = case options[:fontweight]
when 'bold' then 700
else 400
end

#added
text = Array.new
0.upto(4) do |i|
text[i] = Magick::Draw.new
text[i].pointsize = options[:fontsize]
text[i].font_weight = 300 #options[:fontweight]
text[i].fill = 'blue' #options[:color]
text[i].gravity = Magick::CenterGravity
text[i].rotation = (rand(2)==1 ? rand(30) : -rand(30)) if options[:rotate]
end

metric = text[2].get_type_metrics(self.string)
#add bg
canvas = Magick::ImageList.new
fill = Magick::HatchFill.new('white','lime')
x = metric.width+options[:padding]
y = metric.height+options[:padding]

#ADDING LINES

img1 = Magick::Image.new(x,y,fill)
gc = Magick::Draw.new
gc.stroke_linejoin('round')
gc.stroke('blue')
gc.stroke_width(2)
gc.line(rand(x),rand(y),rand(x),rand(y))
gc.line(rand(x),rand(y),rand(x),rand(y))
gc.line(rand(x),rand(y),rand(x),rand(y))
gc.draw(img1)
canvas << background_color =" '#000F'" y_loc =" rand(y)" p =" Magick::Pixel.from_color(options[:background])" opacity =" Magick::MaxRGB" background_color =" p" image =" canvas.flatten_images.blur_image(1)" dir =" self.dir," filename =" self.filename)" downcase ="=">public/images.
def file_path
File.join(dir,filename)
end

class <<> c.created_at+c.ttl
if File.exists?(File.join(WRITE_DIR, c.file_path))
begin
File.unlink(File.join(WRITE_DIR, c.file_path))
rescue Exception
end
end
end
}
end
}
super
end#prune
end#class << image="(i)" image =" i">

visit Here to view the customized use of the plugin validates_captcha in RoR.