ifchanged

the ifchanged tag outputs its contents only if the last call to ifchanged rendered different output.
Example for rendering a list of elements with alphabetical headings:

{% reorder elements by title %}
{% for element in elements %}
  {% ifchanged %} 
    <h3>{{ element.title | truncate: 1, "" }}</h3>
  {% endifchanged %}
  <a href="{{ element.url }}">{{ element.title }}</a><br>
{% endfor %}

Result:

A

An element
Another Element

BBest element ever