Brent Sordyl's blog

Thankfully, Hazel is becoming my electronic assistant that just takes care of these tasks for me. Simply put, Hazel takes the idea of mail rules and applies them to the entire system. At its core, Hazel is a watchdog. You tell it to keep an eye on a folder, and if certain criteria are met, perform an action. We’ve covered Hazel before, but I wanted to focus specifically on combining it with Dropbox.
(Link: How-To: Hazel + Dropbox = Automated File Management)