I usually sell them, and in fact am selling a Nikon D610 full frame DSLR on eBay
If I don't do this, they usually collect dust.
Other great things to do with old cameras is modify them for infra-red, donate them to PhotoStart
to help children learn about photography, or donate them to schools around your community. I've done all of that before as well except for modifying it for infra-red. Still need to do that at some point!!!