I've had all the ports on my MacBook bar the line in use once a few months back:

Power: Duh!

Ethernet: Crossover cable to a customer's laptop, with the MacBook acting as a bridge to my AirPort Express, which is a bridge to my ethernet network

Mini-DVI: 15" SVGA CRT

FireWire: Recharging iPod

USB 1: Mouse

USB 2: USB drive

Line In: Never used it

Line Out: Earbuds

Also done pretty much the same thing with my PB1400 a few times:

Power: Duh!

Line in: Line in plug for AppleVision 1710AV

Line out: Line out plug for AppleVision 1710AV

Video: AppleVision 1710AV

SCSI: External CD-R burner

ADB: AppleVision 1710AV, AppleDesign Keyboard, ADB Mouse II

Serial: LocalTalk networkage