Apart from the modem port and the Audio In port, usually my PowerBook G4 has all the ports in use.

Ethernet is connected to a 5-port switch which is the core of the wired portion of my home network.

FireWire is connected to an external disk (to which the iSight is connected in turn).

USB #1 is connected to another external disk (it has a partition for Time Machine and a partition for the iTunes Library).

USB #2 is either used for iPhone / iPod connection, or to connect another external disk I use as a media centre.

Mini-DVI is connected to an external non-Apple 20-inch LCD monitor via a Mini DVI-to-VGA adapter.

Audio Out is connected either to my external speakers (the beautiful Apple Design Speakers) or to my headphones.

Now, for the 'invisible' ports:

AirPort is connected to my wireless home network and Bluetooth is used with the Apple Wireless Keyboard, Mighty Mouse, cellphones, Newton, etc.

Having all the ports occupied probably saved my PowerBook once when I spilled some orange juice nearby.

Cheers,

Rick