1. Google Code Prettify
2. Google Syntax Highlighter for WordPress
3. Syntax Highlighter Enscript

I prefer Syntax Highlighter Enscript because it’s server side. Here is an example:

<?php
$numbers = array(1,2,3,4,5,6,7,8);
foreach ($loopy = $loop) {
    echo $loop;
}
?>

The other 2 are javascript based but seem to work ok… the only issue I have had is with 3 column themes where in cases of a long line of code it doesn’t wrap.