do you need something more advance than the scan tool inside of Network Utilities (inside the Applications->Utilities folder)?

Are you looking to watch your own port activity or the active ports of another computer? There used to be a program, I think was called Ctanide but I couldn't find it anywhere, that would do all sorts of crazy port things. But I'm not sure if it is still around, since it could be used for a DOS attack pretty easily.

I would also check out http://www.macorchard.com/network/

lots of good apps there.