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ModSecurity is a Web Application Firewall for Apache. It can monitor all of the traffic that is seen by your web server, including request headers and GET and POST data, and block dodgy requests. ModSecurity itself is actually just a rule engine; the clever part is in the rules you pass to it. Many people use the Open Web Appplication Security Project's (OWASP) Core Rule Set (CRS), an open source set of rules that ModSecurity can use to sift the wheat from the chaff, and foil some common types of attack.
The CRS was written by studying known vulnerabilities and writing rules that would not only have prevented the attacks, but prevented other similar attacks too. Thus, ModSecurity provides a good all-round protection for your web server.