SpamCheetah is a network-level spam filtering solution built around OpenBSD greylisting, tarpit, and blacklisting. It comes with a Web interface and requires zero administration. It does not allow spam to even land in your network, thus saving you precious bandwidth. It does not have the 'false positives' problem of losing legitimate mail.
DynaBlast is a fast Apache HTTPD access_log scanner. It can find hosts that exceed a given number of hits in a given time period, printing them to standard output. It has a "fast mode" that can skip lines that have already been parsed. Multiple dynablast instances are avoided by using a lock file. This make it suitable for crontab usage. It can be run by any user, not just root. It can run in parallel, for example, when analyzing different access_log files. It keeps memory consumption down to about 10 kb, by parsing the access_log file one line at a time.