A simple excerpt filter for use with Jekyll. Perhaps there are better ways but I couldn't find anything suitable. It extracts post content in between "<!-- excerpt start -->" and "<!-- excerpt end -->". Change the excerpt delimiters to suit your needs.

2012-03-03-demo.md

layout	title
post	Test Post

This is a test post.

This content comes after the excerpt.

excerpt.rb

# This goes in _plugins/excerpt.rb
module Jekyll
  module ExcerptFilter
    def extract_excerpt(input)
      input.split('<!-- excerpt end -->')[0].split('<!-- excerpt start -->')[1]
    end
  end
end

Liquid::Template.register_filter(Jekyll::ExcerptFilter)

index.html

---
layout: default
title: "Main Page"
---

<div class="posts">
  {% for post in site.posts limit:5 %}
    <div class="post">
      <div class="title"><a href="{{ post.url }}">{{post.title }}</a></div>
      <div class="date">Posted on {{ post.date | date: "%B %d %Y" }}</div>
      <div class="excerpt">{{ post.content | extract_excerpt }}</div>
      <p><a href="{{post.url}}">Read more »</a></p>
    </div>
    <hr />
  {% endfor %}
</div>