Hazel is a tools for sorting files and folders. For alternatives, see here.
Hazel is a System Preference pane that works silently in the background, automatically doing basically whatever you want with the files/folders you specify. It has really useful filters for deciding which items to act on and a lot of options what to do with those items. Move them, rename them, open an Applescript, etc.
Ranked in these QuestionsQuestion Ranking
Pro Automatically cleans up and maintains folders
Hazel watches whatever folders you tell it to, automatically organizing your files according to the rules you create. Have Hazel move files around based on name, date, type, what site/email address it came from (Safari and Mail only) and much more. Automatically put your music in your Music folder, movies in Movies. Keep your downloads off the desktop and put them where they are supposed to be.
Pro Very customizable
More so than alternatives.
Pro Cleans up after uninstalling an application
When you delete an application Hazel will pop up and show you a list of attached files belonging to the deleted app to clean your uninstall more correctly.
Pro iLife Support
Hazel features new actions to import your files into iPhoto or iTunes. And with Hazel 3, you can import into Aperture projects and folders as well.
Pro Process files depending on their content
Its in-file search criteria allows it to extract dates from files (e.g., to add to the filename) and to categorize recurrent files (e.g., receipts) into subfolders (or to treat them in some specific way).
Con Doesn't work well with subfolders
The rules don't work very well with subfolders and working with archive files (zip, rar, gz, 7z, etc) is also limited
Con Rules creation is a time consuming process
To really enjoy the power of Hazel, you will need to progressively create more and more rules to manage each specific kind of file, but this process is slow, quite annoying when you have many similar rules, and can only be learned on the way, as you find new uses for it.