Craigalytics is a project mainly written in Ruby, it's free.
This is a gem to search craiglist rss feeds for keywords and count them, presumably for the purpose of creating fancy graphs.
This gem allows you to count the number of occurrences of specific keywords across craigslist pages using their rss feeds.
You may notice that it doesn't exist yet, and then you will go read this article: http://tom.preston-werner.com/2010/08/23/readme-driven-development.html and everything will become clear.