People commonly overlook the fact that it is not just the desktops and servers that are potentially running unnecessary services—your network devices are also likely doing this. However, here is a list of services you should look for on your network equipment and turn off if you are not actively using them:

• Cisco Discovery Protocol (CDP)
• TCP and UDP small servers
• Finger server
• HTTP server
• Bootp server
• Network Time Protocol (NTP) service
• Simple Network Management Protocol (SNMP) services
• Configuration auto-loading
• IP source routing
• Proxy ARP
• IP directed broadcast
• IP unreachable, redirects, and mask replies
• Router name and DNS name resolution services

In order to find out what services are running you could use ps aux.

To see which ports are open you could use: netstat -an | grep LISTEN

Well if you are Gnome desktop user and would like to have a visual view of the services running and ability to shut them off, you could to System –> Administrator –> Services