I wrote a WordPress plugin in about 4 hours to run a website listing web hosting company reviews. Simple affiliate stuff. The plugin was generic, it would add star ratings to WordPress comments and display average ratings for the post and such. I used the plugin to run my own site, then also built it a little site to sell to anyone else that wanted it. In 18 months it sold over $200,000 in licenses before I sold rights to the plugin to another individual for $90,000.