I have a G4 Powerbook 800 Mhz I bought in 2000 or so. It has been used and abused, including being backed over by a car, but it still runs (mostly) great and, in fact, I am typing on it right now. Between the hardware and the flexibility of OS X, I have made this system useful long past its normal life.
The greatest advantage of this laptop is its large number of built-in ports and communication options: Firewire, USB, ethernet, modem, Airport, S-video, audio in and out, etc. This has allowed me to slowly enhance the system and replace components from the outside as they have failed on the inside.
The brackets holding the screen failed about a year ago. This is a common failing with this model and one I am upset about, but, honestly, going over it with car probably didn't help. Repairing it would cost about $900. On the other hand, using the ADC-to-VGA adapter, I connected an external monitor (free) and turned it into a desktop. Contrary to a lot of opinions, the PowerMac will run just fine closed as long as it is closed before the display is initialized. A USB Mac keyboard (also free) and a graphics pad (not free) turns it into a decent art studio. The keyboard contains a USB hub, so I have no cable mess and have an convenient port on the keyboard tray for my digital camera or, at night, my USB LED light.
It is still a laptop, though, and is stingy on power. The battery makes a great UPS: as long as the power comes up in an hour or two, I'm fine.
I wanted more hard drive space and a convenient backup, so I attached an external Firewire drive. The drive is bootable with OS X Tiger (10.4) which I have gradually upgraded to from the 10.2 which came with the laptop. It runs 10.4 fine and will probably run 10.5 as well. How many people can go from ME to Vista with the same computer? The bootable external drive is great for repairs if I have any problems with onboard disk.
Since the MAC will boot in target mode (acts like a Firewire HD), I use that to make fast transfers to a Linux box (yes, Linux can read HFS volumes) that would take too long over Airport.
Now the CD-RW/CD-ROM/DVD-ROM drive is finally starting to get flaky, so I am looking at a Firewire DVD-RW to add to the Firewire chain.
I use the video out to pipe movies to my TV. I can either use the S-video out port or the included S-video to RCA converter.With VLC, a free video player, I can send the video fullscreen to any attached device and can even display on the monitor and TV at the same time so can see it from the kitchen. Audio, of course, is piped to my Sony receiver so I can use iTunes with my stereo. With a little (actually quite a bit of) playing, I can also share my iTunes library with the Linux box over the wireless connection.
At the moment (just moved) I have dial-up, which I can share to the Linux box over Airport, Firewire, Ethernet, or USB.
The wonderful thing about all of these upgrades is they are all external and can be moved right to my next computer (possibly a Mac Mini) when this one finally dies or I get tired of it. Even as an 800 Mhz G4 (768 MB RAM), though, it still does all of the jobs I need it to do (except the latest FPSes). None of the Intel laptops I have ever had have gone this long and still been this functional.

