People often think there are only two options for old Mac laptops: sell them or throw them away. But there's a third option that can keep parts of your old MacBooks useful for a long time -- a hard drive enclosure. An enclosure allows you to turn any internal hard drive into an external one.
I recently used the Mercury On-The-Go-Pro from OWC. It has allowed me to turn an internal 500 GB hard drive from a MacBook into an external USB 3.0 / Dual Firewire 800 drive that's useful on the lastest MacBook Pro with Retina display.
I literally got the Mercury On-The-Go-Pro in the post this morning and 20 minutes later I was using my old internal drive as an external one. Keep in mind, I had already removed my hard drive my old laptop. But once that was done, converting it into an external drive with the Mercury On-The-Go-Pro was a snap. I simply inserted the hard drive into the enclosure's motherboard, then screwed it in place to the Mercury On-The-Go-Pro's clear acrylic enclosure. I then plugged my new external drive into a USB 3.0 port on my Mac and it transferred data at full USB 3.0 speeds.
Besides the USB 3.0 port, the enclosure offers two FireWire 800 ports, which allows you to have quick transfer speeds on older Macs that lack USB 3.0, but still have the faster-than-USB 2.0 FireWire 800 ports. The best thing about the Mercury On-The-Go-Pro too is that it's bus-powered, so there's no need for an external power supply.
The Mercury On-The-Go-Pro FireWire 800/USB 3.0 enclosure is Mac and PC compatible and costs US$64.99. A USB 3.0-only model is $37.99.