In a GNU/Linux system, (in general, a Unix system), there exists a lot of ready-to-use daemons used to do real common tasks. The most notable are:
Apache and other similar tools: These are the Hypertext Transfer Protocol (HTTP) server daemons
inetd and xinetd: These are the Internet super server daemons
named/bind and other similar tools: These are the Domain Name System (DNS) server daemons
nfsd, lockd, mountd, and statd: These are the Network File System (NFS) daemon and support daemons
ntpd: This is the Network Time Protocol (NTP) service daemon
mysqld, postgresql, and other similar tools: These are the database server daemons
sendmail, exim, postfix, and other similar tools: These are mail transfer agent daemons
snmpd: This is the simple network management protocol daemon
syslogd and other similar tools: These are the system logging daemons