Uses For Old Mac Mini
Use Old Mac Mini As Apple Tv
A few months ago my old Titanium had some problems. Bottlenecks, spinning beachballs, etc. So I got a copy of Scannerz and ran a test on the unit which told me the drive was shot. For those interested, because of it's age, I made the following post some time ago about whether or not I should consider putting an SSD into it: I ended up getting a newer MacBook to replace the Titanium.
I got the MacBook because they were cheap and they could use newer stuff, like SATA drives or SSDs and newer OS X versions. However, the Titanium still works - except the drive. Last night I took an old backup drive I had, connected it to the Titanium, and installed Leopard on it. Now, because it's using an external drive to boot from, it's not really a mobile computer anymore, but it's still working just fine. 
It dawned on me why not put this thing back to use as an NFS network drive? Photo booth for the mac. The MacBook isn't my main computer (excluding the MacBook and Titanium, I have a newer iMac and MacBook Pro). Too often I end up transferring stuff from one system to another using USB flash drives, which is sort of annoying, or even worse yet, having to sync all my stuff that I may need on all systems from one system to all the others.
Uses For Old Mac Mini
You don't need a fast Mac for these tasks; I use a five-year old Mac mini as my server (the only requirement is that it be able to run El Capitan and have at least 2 GB RAM). It's inexpensive, easy to set up, and offers a lot of advantages. You can browse with it, use it as a Kodi/Plex server, or put windows on it and run emulators, probably only up to SNES though. It might even run some PSX games. They have the PSX emulators pretty well optimized now.
Uses For Old Imac
Putting the Titanium back to work to function as a network drive now seems like it's given the old boy new life. This in turn got me wondering about finding even more uses for old computers.