What to do with old devices that are not used anymore?

I - like many people - have a number of devices that I don't use anymore. Things like iPhone 4s, iPad 4, some htc and Nokia devices, an old PC, etc.

I'm currently using my iPad 4 as a digital clock! I made a stand for it and I keep it charged all the time as the screen is always on.

But it bothers me that so much computation power is being wasted. I think old devices still can do something, esp. since most of their components still work.

I was thinking about running my website on the iPad 4, or running my personal VPN through some android devices. What other use cases have you come up with for your old devices?

As with the iPhone 4s, maybe I should just sell it to someone who collects these things at a good price!