Taking apart old electronics
The most valuable thing you can get from discarded electronics is understanding.  Try fixing them, or at least trace out the circuit until you can discover what is wrong.

It used to be very hard to get schematics, but now we have the Internet.  Sites like http://eserviceinfo.com and http://ko4bb.com/ have service manuals for a variety of equipment.  Even if you can't find the manufacturer's service manual, you can often find datasheets for the important components.  Many designs don't stray too far from the datasheet example circuits.

Of course, you can learn about circuits from just reading datasheets and textbooks, but having the physical circuit in front of you gives another level of understanding.