MBP - Apollo, keeping in line with my sky themes.
iMac G4 - Flagship (also my WiFi network name).
PowerMac G4 - Clockwork. Its secondary internal HD is Vault.
PowerBook G4 - Mercury, or informally, Sir Woz (due to the fact it's signed by Steve Woz).
My 20GB 3G iPod is Aurora.
My 250GB FW External is Horizon.
It's a bit funny when I actually reference one of my machines by the above names and a few select friends know what exactly I mean... Things like: "Hey, I used Clockwork last night for all this Photoshoppin'. Gotta Love Panther's speed on that tower!"