I would like to find an FTP client for linux (preferably GTK, but if it's for KDE and does what I need, then I'm fine with it) with very good synchronization features. What I want to use this for for is synchronization of local websites to their online production versions. I know there are command line tools, which works fine, but I'm looking for a GUI roughly like Total Commander has.
This is not an ftp-client, but rsync has many GUI:s and it can operate over ftp. http://www.debianhelp.co.uk/rsyncweb.htm lists many of them. I have no personal experience about those tools, but this might give you a good starting point.