How can I show just the most recent post on my home page with jekyll?

This can be accomplished through the use of limit:

{% for post in site.posts limit:1 %}
... Show the post ...
{% endfor %}

You can also use limit and offset together to “feature” your most recent post:

<h1>Latest Post</h1>
{% for post in site.posts limit:1 %}
... Show the first post all big ...
{% endfor %}
<h1>Recent Posts</h1>
{% for post in site.posts offset:1 limit:2 %}
... Show the next two posts ...
{% endfor %}