I have on hand and use on and off a G4 Digital Audio Powermac that's been upgraded with a Sonnet G4 1.4GHz Processor and boot from a 500GB SATA I/II Hard Drive that's attached to a bootable LaCie 2-port SATA PCI card. I also have another internal 80GB IDE drive that still boots OS 9.2.2.
My other legacy Apple computer(s) that I also use rather frequently for specific tasks - include a G4 Mac Mini 1.25Ghz - which is I believe the very first Mac Mini produced. I use it exclusively as my home networks music server.
And last, but certainly not least, I'm a very proud owner of a mint condition, fully tricked out 20" G4 iMac that is running on a 64GB Kingston SSD (easily accomplished by utilizing a IDE-to-SATA adapter), has a 8x DVD-/+RW DL Optical drive, and bluetooth capabilities provided by a Belkin usb adapter paired with both a Apple wireless keyboard and Magic mouse. Also included in this set-up are a pair of excellent Harmon Kardon Apple Pro Speakers. Everything is working beautifully, with many people who see it in full operation being especially impressed that it's 20" LCD is still amazingly crisp and vibrant - floating effortlessly in space and time.