One of my very first was Inna domain parking app, that it tried to "fertilize" any domain that was pointed to the server. It would obtain feeds piped via Yahoo feeds for keywords from the domain, and sell the domains later at higher price for the slightly better SEO value.

I still love the domain parser I had there. I actively tried to avoid regex by the time, and I had a huge validator that brings tears to my eyes. I admire it because this can be written with a single regex, but I am sure I spent an hour or two working on this, carefully planning about public suffixes, IDN conversion, host headers, etc.