Suricata IDS is a free intrusion detection/prevention system and network security monitoring engine. This is a list of awesome things that go with it.
iblocklist.com makes available many lists of IP addresses in several formats that can be dropped into firewalls or applications to prevent connections attempts from those hosts. Among the lists are known spammers, spyware servers, open proxies, advertising services, governments, and anonymizing services. Useful for perimeter security.