You can use Fish or SFTP to transfer files between computers, with minimal prior setup. Both protocols transfer files over SSH, which is secure and encrypted. They are very well integrated into KDE: you can type
sftp:// URLs into Dolphin's Location Bar, or you can use the "Add Network Folder" wizard. SFTP at least seems to be supported by Gnome too.
I personally use Fish.
On the server machine Fish and SFTP need only an SSH server running, that you can also use to administrate the server machine. Everyone who wants to access the server over Fish or SFTP needs a user account on the server. The usual file access permissions apply, for files accessed over the network.
Fish and SFTP are roughly equivalent to shared directories on Windows, but both work over the Internet too. Usual (command line) programs however can't see the remote files, only programs that use the file access libraries of either Gnome or KDE can see them. To access the remote files through scripts, KDE has the
For a setup with a central server that serves both user identities and files look at NIS and NFS. Both are quite easy to set up, especially with the graphical installers from Opensuse. This is the setup where every user can work at any machine and find his/her personal environment. However the client machines become unusable when they can't access the server.
Furthermore a simple NFS installation has very big security holes. The local computers, where the users sit, have to handle the access rights. The NFS server trusts any computer that has the right IP address. A smart 12 year old kid with a laptop can get access to every file, by replacing one of the local machines with the laptop and recreating the NFS client setup (which is easy).
Off course there is Samba, which has already been mentioned by Grokus. It seems to be quite universal: It can serve files, printers, and login information. It is compatible with Windows and Linux; there is really a PAM Module (Winbind) that lets Linux use the login information form a Samba or Windows server. Samba (and Windows) does not have the security problems of NFS, it handles user identification and access rights in the server.
(Please note: I did never administrate or install a Samba server.)
My conclusion: Fish or SFTP are IMHO best for usage at home. Use Samba if you have Windows clients too. NFS is only useful if you can trust everybody, but I expect it to create the lowest CPU load.