While booting in your Linux box you may have noticed that a lot of services are starting and therefore the boot process is a bit slow. Here i'll show you how to enable/disable system services and which ones are essential for your system. Services names are taken from Fedora Core 6 but most of them are common for all Linux distros.

First of all some things about runlevels. Each one has a different purpose. In most Linux distributions runlevels are organised like this

iD  Name                                          Description
1  Single-User Mode                        Does not configure network interfaces, start daemons, or allow non-root logins.
2  Multi-User Mode                          Does not configure network interfaces or start daemons.
3  Multi-User Mode with Networking  Starts the system normally.
4  Unused  
5  X11                                            Runlevel 3 + X Window System.

To find out how runlevels are organised in your distro take a look in
http://en.wikipedia.org/wiki/Runlevels

In order to determine the runlevels in which a service is on or off we type
chkconfig --level 2345 service on/off

Now we have to find what services are running in our system. The following command gives us all the necessary information
/sbin/chkconfig --list

Of course in each distro there are graphical intefaces doing this job. In Fedora Core/Red Hat systems you can type
system-config-services
or go to [System menu] > Administration > Server Settings > Services

From menu check Edit Runlevel menu -> Runlevels All to change runlevels 3, 4 and 5 at once.

Below there's a list of the most common services in Linux distros. After each service a brief description of the latter is given along with it's recommended status. Please have a look at this description before setting it to the recommended value since your needs may be different than mine.

* acpid
It controls system's power management
Status: Enabled

* anacron
Runs cron jobs
Status: Enabled

* atd
Similar to cron
Status: Disabled

* autofs
Automounts filesystems on demand
Status: Enabled

* ahavi-daemon
Zeroconf stuff
Status: Disabled

* avavi-dnsconfd
DNS Zeroconf stuff
Status: Disabled

* bluetooth
Needed only if bluetooth devices are present
Status: Disabled

* cpuspeed
Dynamic cpu speed daemon.
Status: Enabled only if your cpu supports throttling

* crond
Scheduled tasks
Status: Enabled

* cups
Controls Common UNIX Printing System
Status: Enabled if printer is present

* dhcdbd
D-BUS control of the ISC DHCP client
Status: Disabled

* diskdump
Create dump file if system crashed
Status: Disabled

* firstboot
Quite obvious
Status: Disabled (You have already done your first boot :P )

* gpm
Mouse support in terminal
Status: Disabled

* haldaemon
Daemon for collecting and maintaing information about hardware
Status: Enabled

* hplip
HP Printer Service
Status: Enabled only if you have an HP printer or scanner

* iptables
Firewall
Status: Enabled

* irda
Controls infrared adaptors
Status: Enabled only if an infrared adaptor is present

* irqbalance
Spreads the load across the cpus on a multiprocessor system
Status: Enabled if a multriprocessor cpu is present

* kudzu
Checks for new hardware.
Status: Disabled (Enable only if you change your hardaware)

* lm_sensors
Monitors motherboard sensor values.
Status: Enabled (all new motherboards have sensors)

* mcstrans
Starts the SELinux Context Translation System Daemon
Status: Enabled only if SELinux is enabled

* mdmonitor
Software RAID monitoring and management
Status: Enabled only if RAID is present

* messagebus
System messenger dbus
Status: Enabled

* netdump
Netconsole and netcrashdump utility
Status: Disabled

* netfs
Mounts and unmounts network filesystems
Status: Enabled

* netplugd
on-static network interfaces management
Status: Disabled

* network
Activats/deactivates network interfaces
Status: Enabled (Unless you have nothing to do with ethernet and wireless networks)

* nfs
* nfslock
File sharing between Linux boxes
Status: Disabled (Unless you have a Linux-Linux LAN)

* nscd
Handles passwd and group lookups for running programs
Status: Disabled

* ntpd
Network Time Protol
Status: Disabled

* pcscd
Resource manager for the PC/SC lite and Musclecard frameworks
Status: Disabled

* portmap
Manages RPC connections used by protocols such as NFS and NIS
Status: Disabled (Unless you use the above protocols)

* postfix
Mail Transport Agent, which is the program that moves mail from one machine to another.
Status: Disabled

* rdisk
Discovers routers on the local subnet
Status: Disabled

* readahead_early
* readahead_later
Loads into memory programs before they are needed
Status: Enabled

* rpcgssd
* rpcidmapd
* rpcsvcgssd
NFSv4 helpers
Status: Disabled

* saslauthd
Handles plaintext authentication requests on behalf of the cyrus-sasl library.
Status: Disabled

* sendmail
Mail server
Status: Disabled

* smartd
Self Monitoring and Reporting Technology (SMART) Daemon
Status: Enabled

* sshd
SSH server
Status: Disabled (Unless needed)

* syslog
System logger
Status: Enabled

* vncserver
Remote desktop view
Status: Disabled