Here's something we haven't done for a while, but someone did submit a good question, and it deserves a good answer.

Q. Where do you find the best parts to scavenge for robotics?

A. Just as the best food for fish is fish, the best parts for robots are robot parts. However, finding a dead robot from which to scavenge parts can be difficult under normal circumstances. So I have prepared this list, based on my own experience, of things one is likely to find in normal circumstances, and of what useful things one might find inside those things.

Computers

Power Supply

Not worth scavenging parts from as it contains large capacitors which can be quite dangerous. However, AT/ATX power supplies are quite useful to the experimenter as they deliver a useful range of voltages: +12, +5, -5, -12, and 3.3 volts. If you're using an AT power supply, ensure that the switch is adequately insulated to avoid an electrocution hazard. You will also need to supply a load for the power supply to work properly, such as a large resistor. Perhaps you might consider building an enclosure for the switch, load, and power supply with spring-loaded terminals for each voltage. Although I do not recommend opening the power supply, this page is still helpful. And here are some other ways to get power from a PC.

Motherboard

Very old motherboards are treasure troves of 7400-series chips, kilobytes of memory, and some specialty chips like UARTs and processors. However, unless the board is bad, first consider whether it might serve you better as the brain of your robot.

Expansion cards

As with motherboards, very old expansion cards are even bigger troves of 7400-series chips. People are usually happy enough to get rid of their old modem cards, and these usually have relays, which are useful for controlling motors, especially larger ones.

Floppy disk

I don't get too excited about 3.5" floppy disk drives as the stepper motors are pretty small and haven't proven that useful when removed. On the other hand, 5.25" drives, if you can still find them, have much larger steppers. You may find a stepper motor driver on the board if you're extremely lucky, but in my experience, the steppers are driven by the drive's CPU either directly or via a transistor array. You might also, with some research, be able to get the pancake motor (that spins the disk) to work. On the oldest 5.25" drives, the flashing of an infrared beam shining through a hole punched in the disk was used as a timing signal to keep the motor spinning at 300 RPM. Other pancake motors use magnets instead of a beam of light to accomplish this.

CD-ROM

You can usually find two to three DC motors in a CD-ROM drive: one to spin the disk, one to move the laser, and one to move the tray. You'll also find some gearing assemblies in conjunction with the tray that may or may not be useful in making a linear actuator. If you look on the board, you could find a few audio amplifiers, a DC motor controller, or a microcontroller.

Hard disk

Unless it's very old, there won't be a lot of useful parts except for the neodymium magnets. These have all sorts of uses, particularly in attaching heavy objects (like bulletin boards) to metal objects (like your refrigerator.)

Mouse

You'll find a few microswitches, and either a pair of rotary encoders with infrared emitters and phototransistors, or a tiny camera.

Printers, fax machines, copiers*

Here's where you'll find a few motors and loads of gears, cams, rods, springs, and pulleys. In inkjet and dot-matrix printers, you'll also find a screw or belt that drives the print head back and forth. There will probably also be an encoder disc or even a strip that the printer's CPU uses to ensure that the print head is in the right horizontal position.

Entertainment

Stereo*

Loads of analog components. If it's got a CD player, see CD-ROM. If it's got a tape player, there will be at least one DC motor and perhaps a belt system. You may also find a DC motor controller.

VCR*

You'll probably find at least one DC motor and motor driver. You'll also find a few gears and belts, and an infrared detector.

Televisions (and monitors)*

Not much besides a few analog components such as resistors and capacitors. In fact, old televisions often have especially large capacitors inside, which can be dangerous if not properly discharged. I would avoid these entirely.

* Take care when opening such devices. Unlike your friendly neighborhood PC, the power supply in one of these devices is unlikely to be shielded, and even if the device has been turned off and unplugged, large capacitors can store a hazardous amount of voltage for a surprising amount of time.