A simple FTP uploader plugin that will attach and and push new/changed files to an FTP server.
Still thinking about what should happen to deleted files, should they also be removed on the FTP server? The plugin will have similar functionality to the emailer with regard to options. Need to be careful about performance when there is a high volume of events. Errors can easily occur as well if files go missing during uploads or connectivity becomes a problem.
Antti N commented
This is great idea but it DEFINITELY needs SFTP. FTP is way too insecure for anything and many serious servers don't even support it anymore.
This can be done already now without any plugins, just scripting but this should be plugin that holds custom profiles for sites (incl. base directory) and then user can select which profile to use and get relative path from detected files.
The main idea would be that you don't need anything else, just go to Directory settings in DM and in Execute select option 'copy to server' or something like that. Then just set up the above things and all is done :)
In short, normal SFTP client with DM integration :)
If you need this functionality now you can use the Execute Plugin with a batch script to upload. Details of batch scripts that upload files to FTP can be found here: http://www.dostips.com/DtTipsFtpBatchScript.php
PowerShell is also a good option for something like this.
If scripting is not your thing then the plugin for this will simply the process of uploading to FTP.