Can you not use the text widget (if you're using wp that is) ?
there are plenty of plugins that you can hack to do what you are asking. most of the plugins that do what you are asking use the the_content filter hook and look for a tag (i.e. <!--insertcodehere-->) and do a simple php str_replace
<?php
/*
Plugin Name: My Custom Content
Plugin URI: http:**whatever.com
Description: blah
Version: 1.0
Author: Me
*/
function myfunction_insert( $content ) {
if(preg_match('#<!--mycustomcontent-->#', $content)) {
$newcontent = str_replace('<!--mycustomcontent-->', 'my custom code right here', $content);
}
return $newcontent;
}
add_filter('the_content', 'myfunction_insert');
?>