This is a nice summary of web application security related technologies, processes, and development patterns: Design Guidelines for Secure Web Applications. A little .NET heavy, but most stuff is generally applicable.

If you read and like the above information, you should not miss the OWASP web security guidelines. This is a must read for every tester and developer. OWASP Guide Project:

Web application security is an essential component of any successful project, whether open source PHP applications, web services such as straight through processing, or proprietary business web sites. Hosters (rightly) shun insecure code, and users shun insecure services that lead to fraud. The aim of this Development Guide is to allow businesses, developers, designers and solution architects to produce secure web applications. If done from the earliest stages, secure applications cost about the same to develop as insecure applications, but are far more cost effective in the long run.

Unlike other forms of security (such as firewalls and secure lockdowns), web applications have the ability to make a skilled attacker rich, or make the life of a victim a complete misery. At this highest level of the OSI software map, traditional firewalls and other controls simply do not help. The application itself must be self-defending. The Development Guide can help you get there. The Development Guide has been written to cover all forms of web application security issues, from old hoary chestnuts such as SQL Injection, through modern concerns such as AJAX, phishing, credit card handling, session fixation, cross-site request forgeries, compliance, and privacy issues…