Just thinking about a few of my favourite PowerPC era devices which are still going strong...
I have two different 2004 vintage LaCie 'F.A Porsche' design Firewire DVD burners, one of which had the short lived "Lightscribe" tech. I laser 'scribed a few discs in my time with it. I've often used one of these as a powered Firewire 3.5" HDD enclosure for gaining access to IDE/PATA drives I may otherwise not be able to.
Another device I would be unlikely to let go of is my Elgato EyeTV E410 which is a cool little unit for watching digital TV on the Mac. It sits perfectly atop the G5. Playback and recording is seamless with the EyeTV version 3 software. Playback of the HD channels is not so great on a G4 though.
Another favourite little firewire device is my Canopus ADVC-55 which I have long used as an anologue video input for the G5. It handles real-time encoding of RCA or S-video A/V input. Latency is next to nil which meant it allowed me to play console games (primarily a PS2 at the time) on the G5's cinema display just like a regular TV set. It also allowed easy capturing of those games as well as from the trusty old VCR. It's amazing how many old VHS movies never made it to DVD. I occasionally did audio captures from the old video tapes (often b-grade horror movies from the 70's and 80's) for sampling in music productions.
If you're going for external Apple products which were PowerPC-specific, then an iPod classic (touch wheel, U2 special, etc), an early iPod mini, Shuffle or Nano (or even the original 20GB Firewire brick-pod) would be a perfect compliment. I still use my 8GB Blue iPod Nano on a near-daily basis - somehow the battery is still kicking along just fine.
Another PowerPC-era device which I have relied upon almost daily since 2003 is my original Digidesign Mbox 1. It's a USB1.1 powered audio I/O with two channel XLR/TRS inputs with Mic and instrument pre-amps, phantom power, SPDIF I/O, TRS outputs, TRS inserts and headphone 3.5 and 6.5mm outputs. It's such a humble little device but manages to allow my G5 to process 48Khz 24-bit audio I/O in realtime, with relatively low latency in Pro Tools LE 7.4 and Ableton Live 8. If you're interested in audio production, look around for the Digi 002 or 002R which is an 8 channel Firewire I/O.
M-Audio USB powered Midi performance controllers like the MK49 or Oxygen 8 are great little PowerPC compatible keyboards from the era too.
