If you run Apache Web then head over to Pete Freitag's site to learn some great tips on how to strengthen security. Freitag lists 20 ways to secure your Apache configuration, many of which require only a little of your time.
One important item from the list is to use modsecurity , which is "an open source intrusion detection and prevention engine for web applications" that runs as an Apache module. The latest version, 1.9, was finalized and released on November 30. Are you running the latest version?