How to show only today’s post?

This should do the trick:

<?php
$day = date('Ymd');
query_posts("m=$day");

The query arg 'm' for WordPress will be interpreted as a year if it is four characters long, a month if 6, and a date if 8.

EDIT

You should probably add in the original query string just in case you’re writing enough to need things like pagination:

<?php
global $query_string;
$query_string = empty($query_string) ? 'm=' : $query_string . '&m=';
$query_string .= date('Ymd');
query_posts( $query_string );

That will allow you to preserve other query arguments that are supposed to be there.