Do I need a plugin to load entries via AJAX? I want to load some entries via AJAX. I have a page that normally shows all entries of a specific type with user-selectable filters. I want to load filtered entries via AJAX. 

Do I need to create a plugin to do this? If so, how can I use the Craft object in my JavaScript code to call Craft.ajaxActionRequest or Craft.postActionRequest?
There is a new RESTful plugin which might be what you need: Rest-Easy from Wes Rice.
But you could also make use of Twig's extends functionality:

_ajax.html:

{% block ajax %}
    { success: false }
{% endblock %}


entry.html:

{% extends craft.request.isAjax ? "_ajax" : "_main" %}

{% block ajax %}
    { success: true }
{% endblock %}

{% block normal %}
    &lt;p&gt;Normal entry&lt;/p&gt;
{% endblock %}


If you request entry.html via ajax, the template will extend _ajax.html instead of _main.html, and show your json data. In this example _main.html would be your normal layout-template.

Note: Don't forget to set the XMLHttpRequest in your ajax call.