I have a mashup of Perl, mod_security, and iptables running on my server to drop traffic from hosts I deem bad. I use mod_security and a whole slew of rules to catch hosts doing bad things to my Apache server -- things like comment spamming, referrer spamming, trying to execute scripts/commands and other assorted nastiness. All hosts that get caught doing bad things by mod_security are combed out of the error logs. Then those host IP addresses are fed into iptables with an explicit rule to drop further http packets from these hosts. It is plain and simple.
Anyways, if you want to see the current list of hosts which my server is ignoring, check out the IP addresses or the resolved names. The list resets at the end of the month every two weeks, so if you are being dropped right now, clean up your machine, and mine will welcome you back next month in a few days!