These are random links that might be interesting for Horde developers
+++ Security
[http://www.owasp.org/ Top Vulnerabilities in Web Applications]. It may be most efficient to pick one potential vulnerability and audit the entire code base for it before moving on to the next vulnerability.
[http://www.theregister.co.uk/content/55/29511.html Top 10 e-commerce software flaws]
+++ Performance
[http://www.advogato.org/article/646.html Advogato article on large-scale web applications]
[http://www.zend.com/zend/art/ref-count.php A very good read on how references and reference counting work]
[http://phplens.com/lens/php-book/optimizing-debugging-php.php Some more optimization pointers]
+++ Usability