AjaxFTP is a drop-in FTP widget that you can use in your Web 2.0 applications. It's modular, skinnable, actively in development, and uses the Ruby on Rails Prototype JavaScript library for a seemless FTP browsing experience.

You can get the open source version of AjaxFTP at the SourceForge page

You can buy the commercial version online here, and also see screenshots of it in action.