Favourite pattern: Interception/Filter pattern
Anyone watching sci-fi or war movies knows what an interceptor is.
What it basically does: preprocess a request, filter (any bad) stuff before it reaches the request handler
Applications doing this (in a sort of way): firewalls, proxies, antispam, isapi, rules engines
What I'd like to happen(for security sake): get good APIs at the application level for easier building of such application (think anti-spam in Outlook, anti-phishing in IE)
Here a good link:
http://java.sun.com/blueprints/corej2eepatterns/Patterns/InterceptingFilter.html