common configuration files

bashrc	The system-wide configuration file for the Bourne Again SHell. Defines functions and aliases for all users. Other shells may have their own system-wide config files, like cshrc.
crontab and the cron.* directories	Configuration of tasks that need to be executed periodically - backups, updates of the system databases, cleaning of the system, rotating logs etc.
mtab	Currently mounted file systems. It is advised to never edit this file.
nsswitch.conf	Order in which to contact the name resolvers when a process demands resolving of a host name.
pam.d	Configuration of authentication modules.
passwd	Lists local users. Use the shadow utilities useradd, usermod and userdel to edit this file. Edit manually only when you really know what you are doing.
printcap	Outdated but still frequently used printer configuration file. Don't edit this manually unless you really know what you are doing.
profile	System wide configuration of the shell environment: variables, default properties of new files, limitation of resources etc.
rc*	Directories defining active services for each run level.
resolv.conf	Order in which to contact DNS servers (Domain Name Servers only).
sendmail.cf	Main config file for the Sendmail server.
services	Connections accepted by this machine (open ports).
sndconfig or sound	Configuration of the sound card and sound events.
ssh	Directory containing the config files for secure shell client and server.
sysconfig	Directory containing the system configuration files: mouse, keyboard, network, desktop, system clock, power management etc. (specific to RedHat)
X11	Settings for the graphical server, X. RedHat uses XFree, which is reflected in the name of the main configuration file, XFree86Config. Also contains the general directions for the window managers available on the system, for example gdm, fvwm, twm, etc.
xinetd.* or inetd.conf	Configuration files for Internet services that are run from the system's (extended) Internet services daemon (servers that don't run an independent daemon).