I coded a Craigslist bot that scrapes all the new listings for a specific category you're looking in and auto-emails each of the vendors. It was an interesting exercise in crawling that operates similarly to my C search engine here: https://github.com/deloschang/search-engine.
Why? I'm tired of browsing all the Craigslist postings and emailing each person. I thought it'd be a fun solution to auto-email the vendors, as I was going to email the same thing anyway.
In Python, BeautifulSoup parses the page for the Craigslist links. It saves them to a postgres database so that it won't duplicate-email the same URL. It also "intelligently" remembers the name of the Craigslist posting so if somebody creates a duplicate with the same name to readvertise, it won't double-email them.
I've left it on overnight and it makes browsing Craigslist a lot easier.: