1, Disassemble the computer and the fan to clean out accumulated dust and fluff. Apply fresh thermal paste. Upgrade some components such as RAM and drive to an SSD. Perform clean install of Windows and configure it to run without many visual effects, and disable unnecessary services, all to enable an older computer to run almost as well (and in many cases better) than a new PC. 2. I configured one to be a media server, serving up thousands of music albums that I ripped from my own CDs and some vinyl. 3. "kiosk"-type computer to display videos and slide shows in automated mode in trade show booths. 4. Keep one, improved as in my option 1., to serve as a backup/spare computer in case my primary computer is ever down. 5. Scavenge the components to use to repair or upgrade other old computers.  
Other than recycling, what are some ways you have repurposed an old system? For more questions regarding this contest, please visit the Information Page here.
DS 07/25: Other than recycling, what are some ways you have repurposed an old 
system?