Most up tp date operating systems will not tun on an old pentium one. Openbsd will and makes a great home web server.
Old B&W works well enough with the Raspberry Pi.
Use an old "AT: motherboard serial cable on the Raspberry Pi for a gpio connection.
Turn an old PC into a clock and conversation piece. (http://www.tenox.tc/out/)
Use an old tv as a monitor for the Arduino via tv.out.
Stepper motor from old floppy drive for robotics projects.. (Worth repeating)
Use old hand help pdas and like as serial terminals. Once upon a time it was the hacking device of choice.
Nas appliances used as full servers. You can install linux on quite a few of them.
De-soldering braid from cable shielding. This one was from the vga cable I was making the vga breakout cable.
Linux running on an old Apple Mac G3. (worth repeating)
Most old power supplys including the old xt have +5 and +12 that can be used for projects. (An Amiga ps shown) Check the manual and or documentation to be sure. Even if they are dead , they are great for parts such as heat sinks and etc.
The old Airlink101 430w will run dd-wrt, but you have to do a redboot install (advanced users).
Have an old intel based computer that is still good, but it will not run MSWindows or Linux. Try ReactOS. http://www.reactos.org/
Old floppy cases are great cd rom holders or a great place to store hand written recipes.
Turn your old pc's into thin clients and boot from an ltsp server.https://www.instructables.com/id/Another-almost-free-computers-thin-client-set-up/ https://www.instructables.com/id/PC-personal-computer-into-a-sort-of-thin-client/
Turn an old pc into a network router. (https://www.instructables.com/id/Openwrt-on-a-pc/)
Take and old HP jet direct and use it for home automation.
You can mount your old computers like trophies. (No taxidermist required.)
Dos (yes a dos) web server. http://www.rubbermallet.org/software/webserv.html We have it working!
Old pc = rocket launcher, home automation controller, holiday light controller, or a robot brain! (Boots from compact flash).
Turn an old pc into a stereo. Another use for that old server. (We just ssh into it and run our playlist or just play a random song). (mpd or mplayer)
Reconstructing old computers into fpga replacements. http://www.youtube.com/watch?v=5uaDzF99a80
Robot clock: http://qrtime.com/ (Can be run from a local web server or in a simple browser locally)
An association that reuses computers: http://rso.cornell.edu/ccra/ .
This site has some interesting uses (i.e. guitar pick): http://smilejunkie.com/stuff-made-of-other-reused-stuff-how-cool-is-that/
Magnets from hard drives as refrigerator magnets
Home made generators from hard drive magnets
Let your computer be a stereo. for older machines that will run the mt-daapd music server.
More to come,