diff --git a/lib/jekyll/converters/markdown.rb b/lib/jekyll/converters/markdown.rb
index 485cac82..178eaaa8 100644
--- a/lib/jekyll/converters/markdown.rb
+++ b/lib/jekyll/converters/markdown.rb
@@ -8,20 +8,22 @@ module Jekyll
 
       def setup
         return if @setup
-        @parser = case @config['markdown']
-          when 'redcarpet'
-            RedcarpetParser.new @config
-          when 'kramdown'
-            KramdownParser.new @config
-          when 'rdiscount'
-            RDiscountParser.new @config
-          when 'maruku'
-            MarukuParser.new @config
+        @parser =
+          case @config['markdown'].downcase
+            when 'redcarpet' then RedcarpetParser.new(@config)
+            when 'kramdown' then KramdownParser.new(@config)
+            when 'rdiscount' then RDiscountParser.new(@config)
+            when 'maruku' then MarukuParser.new(@config)
           else
-            STDERR.puts "Invalid Markdown processor: #{@config['markdown']}"
-            STDERR.puts "  Valid options are [ maruku | rdiscount | kramdown | redcarpet ]"
-            raise FatalException.new("Invalid Markdown process: #{@config['markdown']}")
-        end
+            # So they can't try some tricky bullshit or go down the ancestor chain, I hope.
+            if allowed_custom_class?(@config['markdown'])
+              self.class.const_get(@config['markdown']).new(@config)
+            else
+              Jekyll.logger.error "Invalid Markdown Processor:", "#{@config['markdown']}"
+              Jekyll.logger.error "", "Valid options are [ maruku | rdiscount | kramdown | redcarpet ]"
+              raise FatalException, "Invalid Markdown Processor: #{@config['markdown']}"
+            end
+          end
         @setup = true
       end
 
@@ -38,6 +40,19 @@ module Jekyll
         setup
         @parser.convert(content)
       end
+
+      private
+
+      # Private: Determine whether a class name is an allowed custom markdown
+      # class name
+      #
+      # parser_name - the name of the parser class
+      #
+      # Returns true if the parser name contains only alphanumeric characters
+      # and is defined within Jekyll::Converters::Markdown
+      def allowed_custom_class?(parser_name)
+        parser_name !~ /[^A-Za-z0-9]/ && self.class.constants.include?(parser_name.to_sym)
+      end
     end
   end
 end
diff --git a/site/docs/extras.md b/site/docs/extras.md
index 0c190ec1..14261e34 100644
--- a/site/docs/extras.md
+++ b/site/docs/extras.md
@@ -16,7 +16,12 @@ Maruku comes with optional support for LaTeX to PNG rendering via blahtex
 Maruku to not assume a fixed location for `dvips`, check out [Remi’s Maruku
 fork](http://github.com/remi/maruku).
 
-## RDiscount
+## Alternative Markdown Processors
+
+While Jekyll defaults to using Maruku for Markdown conversion, you may use one
+of the other three pre-defined markdown parsers or define your own.
+
+### RDiscount
 
 If you prefer to use [RDiscount](http://github.com/rtomayko/rdiscount) instead
 of [Maruku](http://github.com/bhollis/maruku) for Markdown, just make sure you have
@@ -34,7 +39,7 @@ have Jekyll run with that option.
 markdown: rdiscount
 {% endhighlight %}
 
-## Kramdown
+### Kramdown
 
 You can also use [Kramdown](http://kramdown.rubyforge.org/) instead of Maruku
 for Markdown. Make sure that Kramdown is installed:
@@ -54,3 +59,34 @@ Kramdown has various options for customizing the HTML output. The
 [Configuration](/docs/configuration/) page lists the default options used by
 Jekyll. A complete list of options is also available on the [Kramdown
 website](http://kramdown.rubyforge.org/options.html).
+
+### User-Defined
+
+So, you're totally at odds with our four built-in markdown parsers, eh? No
+sweat. You can define one as a plugin:
+
+{% highlight ruby %}
+require 'jekyll'
+require 'some_renderer'
+
+class Jekyll::Converters::Markdown::MyCustomParser
+  def initialize(config)
+    @site_config = config
+  end
+
+  def convert(content)
+    # (this _must_ return the resulting String after the rendering)
+    SomeRenderer.new(@site_config).to_html(content)
+  end
+end
+{% endhighlight %}
+
+Once you've got that setup, ask Jekyll to use your custom markdown parser in
+your `_config.yml` file:
+
+{% highlight yaml %}
+markdown: MyCustomParser
+{% endhighlight %}
+
+(Note that this **is case-sensitive**, and is only the piece after
+`Jekyll::Converters::Markdown`.) And there you are!
diff --git a/test/test_site.rb b/test/test_site.rb
index a530f9d0..04e2b3d0 100644
--- a/test/test_site.rb
+++ b/test/test_site.rb
@@ -262,6 +262,52 @@ class TestSite < Test::Unit::TestCase
       end
     end
 
+    context 'using a non-default markdown processor in the configuration' do
+      should 'use the non-default markdown processor' do
+        class Jekyll::Converters::Markdown::CustomMarkdown
+          def initialize(*args)
+            @args = args
+          end
+
+          def convert(*args)
+            ""
+          end
+        end
+
+        custom_processor = "CustomMarkdown"
+        s = Site.new(Jekyll.configuration.merge({ 'markdown' => custom_processor }))
+        assert_nothing_raised do
+          s.process
+        end
+
+        # Do some cleanup, we don't like straggling stuff's.
+        Jekyll::Converters::Markdown.send(:remove_const, :CustomMarkdown)
+      end
+
+      should 'ignore, if there are any bad characters in the class name' do
+        module Jekyll::Converters::Markdown::Custom
+          class Markdown
+            def initialize(*args)
+              @args = args
+            end
+
+            def convert(*args)
+              ""
+            end
+          end
+        end
+
+        bad_processor = "Custom::Markdown"
+        s = Site.new(Jekyll.configuration.merge({ 'markdown' => bad_processor }))
+        assert_raise Jekyll::FatalException do
+          s.process
+        end
+
+        # Do some cleanup, we don't like straggling stuff's.
+        Jekyll::Converters::Markdown.send(:remove_const, :Custom)
+      end
+    end
+
     context 'with an invalid markdown processor in the configuration' do
       should 'not throw an error at initialization time' do
         bad_processor = 'not a processor name'