KDE and GNOME have some system configuration tools. Even if you don't use GNOME, you can still install the gnome-system-tools package that comes with these tools:
/usr/bin/network-admin
/usr/bin/time-admin
/usr/bin/boot-admin
/usr/bin/services-admin
/usr/bin/users-admin
/usr/bin/disks-admin
/usr/bin/shares-admin
You can also install the bum package that provides you with a tool for configuring services (it's like services-admin in gnome-system-tools, only better).
Some Debian packages have post-install configuration scripts and you can rerun them with "dpkg-reconfigure some_package" (where "some_package" is the package you want to reconfigure). There's also a GUI tool, gkdebconf, that allows you to run these scripts from a GUI frontend.
There's a package called wajig that you can install and it's supposed to be a frontend to several config tools. The wajig package also installs a GUI frontend "gjig".
http://www.togaware.com/wajig/
There are also loads of command-line tools, some Debian-specific and some not. When you learn more about your system, you'll almost never need a GUI tool. You just open a terminal window and edit some config files. Most of these files can be found under /etc , especially the files under /etc/default are worth checking out. And every installed package has additional information under /usr/share/doc . Also, most commands have a man page that is sometimes worth checking out.