Many web-based organizations depend on the ability to communicate and share files easily. Even if you already have a website, however, you might not have an easy way to share files and folders, aside from giving each user FTP or SSH access.
There are proprietary solutions, such as Dropbox or even the new Ubuntu One, but if you want to run such a system on your own server, the clear winner here seems to be iFolder by Novell. By default iFolder server is designed to run only on Novell’s SUSE Enterprise Server or an OpenSUSE server, but because it is open source, some have had success building and running it on CentOS and Ubuntu servers.
On the client side, iFolder supports Linux, Mac OS X, and Windows, making it an ideal cross-platform solution. To run iFolder, your server needs to be running Mono, an open source alternative to .NET, which it already should have if it is a SUSE server. If not, it is available for installation on most Linux distributions. iFolder is free software released under the GNU GPL.