From the c-icap homepage:

c-icap is an implementation of an ICAP server. It can be used with HTTP
proxies that support the ICAP protocol to implement content adaptation and
filtering services.

Most of the commercial HTTP proxies must support the ICAP protocol. The open
source Squid 3.x proxy server supports it. 

Major features:

    basic C API for developing custom content adaptation and filtering services
    plugins interface
    LDAP integration
    simple ICAP client API

Currently the following services have been implemented for the c-icap server:

    Web antivirus service, using the clamav open-source antivirus engine
    basic URL filtering service

WWW: http://c-icap.sourceforge.net/