Laptops:
2002 Powerbook G4 667 Mhz, 15": RAM upgraded to 768MB, one of the USB ports is kind of dodgy due to the plastic tab supporting the contacts snapping off, and the battery no longer holds a charge, but otherwise works. Runs 10.4.11, slowly, but usable for web browsing, and emails. - Still working @ 12 years of age.
2006 Macbook Pro Core 2 Duo 2.33 Ghz: HDD upgraded to 500MB, screen hinge somewhat loose, and an unfortunate dent in the back of the case, in otherwise working order. Battery charge lasts about 50 minutes though. Fairly usable for anything except gaming or heavy graphics work. - Still working @ 8 years of age.
Late 2013 Retina Macbook Pro, 2.3 Ghz: Still "new" and in perfect working order
Desktops:
Macintosh 512k E - broken disk controller in internal drive sometime in 1993-4?
1992 LC II - Upgraded the processor with a MicroMac upgrade card to double the speed to 32Mhz, RAM maxed out at 10 MB. - Worked until the logic board failed around 2001.
1997 Power Computing Power Center Pro 180 - RAM upgraded to 64 MB, still boots OS 9.1. Optical drive starting to go, no longer reads most burned CDs. Otherwise going strong ~ 17 years later.
2001 Digital Audio Dual 533 Mhz G4 - HDD added and upgraded to 750GB, 500GB x 2, RAM upgraded to 1GB, processors upgraded to dual 1.6 Ghz G4, video card upgraded to ATI Radeon 9600 AGP 8x 256 MB, running 10.4.11, works perfectly, can even do some light gaming (Halo works well). Going strong ~ 13 years on.