How to check open ports in linux

nmap -sS -O 127.0.0.1

Starting Nmap 4.11 ( http://www.insecure.org/nmap/ ) at 2011-05-08 16:49 CEST
Interesting ports on server.abhi.com (127.0.0.1):
Not shown: 1660 closed ports
PORT     STATE SERVICE
21/tcp   open  ftp
22/tcp   open  ssh
25/tcp   open  smtp
53/tcp   open  domain
80/tcp   open  http
106/tcp  open  pop3pw
110/tcp  open  pop3
111/tcp  open  rpcbind
143/tcp  open  imap
443/tcp  open  https
465/tcp  open  smtps

Device type: general purpose

Running: Linux 2.4.X|2.5.X|2.6.X

OS details: Linux 2.5.25 - 2.6.3 

Uptime 95 days

Nmap run completed -- 1 IP address (1 host up) scanned in 2.341 seconds