http://wordpress.designpraxis.at/plugins/disk-usage/
Your Webspace is overquota? Your backup files have grown massively? You want to know where the really big files are hidden?
Then this is for you: Disk Usage gives you an overview on which files and directories are using up your webspace.
This Plugin is a frontend for the Unix du command. It is just as simple as it can be. No configuration, no options. Advanced version comes with the next release.