A list of commonly-used ports can be found at:
http://www.opendoor.com/doorstop/ports.html

If you see one of the listed ports open, then make sure you realize you are running that service. For example, if a netstat reveals a port listening on 22, you have SSH service running. If you are running this for a purpose, then it's not a problem, but if you did not realize it was running and you don't want it, do as thehorse suggested, and search for the source of the service and how to shut it down.

Another thing is that ports above 1024 are typically used by applications, such as Kazaa, so be aware of that. Make sure you are aware of the port range that your applications use (somewhere in the settings) and if you see something outside that range, a trojan or something else may be on your system.

dont have port 23 there nor 80.
Some of the more common ports will actually be listed as [machinename]:[servicename]
i.e. alastair:http instead of alastair:80, so keep your eye open for those as well.

Good luck!