<?php
<br/>
$block = module_invoke('module_name', 'block_view', 'block_delta');<br/>
print $block['content'];<br/>
?>'module_name' = The machine name of the module (i.e. the module's folder name). This is true for core modules too, so for instance 'search', 'user' and 'comment' would all work here.
'block_delta' = The machine name of the block. You can determine what this is by visiting the block administration page and editing the block. The URL for editing a webform block, for instance, would be something like:
Drupal 7: admin/structure/block/manage/webform/*client-block-11*/configure
In this example, 'webform' is the module's name, 'client-block-11' is the block's delta.
Custom blocks will have module name of 'block' and a number for a delta, which you can also find by editing the block.
More information: http://drupal.org/node/26502