List TCP ports that are listening for connections, and the program that is running the daemon:
netstat -lt --program
Same thing for UDP:
netstat -lu --program
A mapping of service name to port number can be found in /etc/services. Refer to http://www.iana.org/
for more info.
You can also port scan yourself for open ports with a program like nmap (http://insecure.org/
), which may already be installed on your system.
Some ports that get turned on by default that you probably want turned off:
lpd on port 515
portmapper on port 111 (unless you are mounting NFS volumes)
sendmail on port 25
If you want to stop services, and prevent them from starting on boot, here's an example using sendmail:
chkconfig --del sendmail