<p>Does anyone have any recommendations for implementations of ActiveRecord in PHP? I've been using <a href="http://31tools.com/cbl_activerecord/" rel="nofollow noreferrer">CBL ActiveRecord</a>, but I was wondering if there were any viable alternatives.</p>

Depends! ;)
For example there is
ADODB's Active Record implementation
, then there is Zend_Db_DataTable and
Doctrine
. Those are the ones I know of, I am sure there are more implementations.
Out of those three I'd recommend Doctrine. Last time I checked Adodb carried a lot of extra weight for PHP4 and Zend_Db_* is generally not known to be the best in terms of completeness and performance (most likely due to its young age).
Doctrine aside from Active Table and the general database abstraction thing (aka DBAL) has so many things (e.g. migrations) which make it worth checking out, so if you haven't set your mind on a DBAL yet, you need to check it out.