Rumpus is a great FTP product that has a web interface as well. The issue with installing Rumpus on OS X Server is that the default web ports are used by the server itself.
So if you are not planning on using web services or, more importantly, Profile Manager. You can follow the steps in Maxum Development's blog post to disable the built-in apache service in OS X Server. You can then enable Rumpus on port 80 or 443.
http://www.maxum.com/Rumpus/Blog/OSXServer.html