To build stuff, you need stuff! While you could just go and buy what you need, not all of us have the $$$ and besides, you learn more by using what you can get sometimes.

One of my favourite sources for parts is old printers and scanners. The newer they are, the less interesting stuff can be easily had since there has been a shift from printers with several dedicated boards (eg one motor controller board, power supply, processing, interface etc) and a few stepper motors to just one integrated board and a DC motor or two. But even new printers have some useful stuff in them. Here are some of the useful things you can look out for, as well as just pictures from the latest laser printer I took apart:

Stepper motors - by far the most useful parts for me at least, these can be controlled by your computer or arduino or whatever to rotate in small increments. You can find various sizes and strengths, so google around to find the pinouts and stats:

Power supplies. If you're lucky, the printer will have a separate board outputting something like +24V, GND, +5V and possible +12V or something. These are nice for running any motors you scavenge.

DC motors. Some printers use a DC motor and an optical encoder for positional feedback. You can try your hand at working with these for precise control, but personally I just rip the motors out and use them for other things. They are generally quite high torque and decently fast...

Brushless motors. These are super hard to drive, BUT there is usually a motor controller chip right next to them. Google the part number and you can figure out a +V, GND, and GO_AS_FAST_AS_YOU_CAN pin :D These are quite often used to spin the mirror in the laser assemblies of laser printers, but beefier ones are also used to push the paper through in some printers. Here's one with a square mirror Note the piece of paper I keep with it so I don't have to keep looking up the pinout:

Strait rods and slides. If you're planning any sort of mill/3d orinter/laser cutter/xy thingee, these are super useful.

Screws, springs and gears. A lot of printers only use two or three different screws, so they are a good source of parts if you need a lot of the same thing. Chuck the springs and smaller pieces into a jar - they will come in handy when you least expect (see if you can spot one in #Scanning EIT Imaging System (SEITIS)).

Little solenoid/clutch things. Apply a current and the little lever moves and potentially stops a gear from spinning.

Photo-interrupters. Most printers have at least four, with some larger ones having closer to 20! These little things are easy to interface with, and can be set up to triger an event when the beam is broken.

Photo-interrupters, an electric clutch thing and some assorted screws:

A fusor module from laser printers is basically a heating element. Some a quartz crystals, others are basically a long thin ceramic resistor. Don't burn yourself...

Metal and plastic stock. Especially if you're just starting to build a collection of materials, some nice flat pieces of metal with lots of threaded holes and attachment points are useful building platforms. Don't bother keeping everything, but grab some of the nicer looking pieces of metal and plastic to leave in a box until you need them.

Small PCBs with a few buttons and LEDs. Need to add an interface to something but can't be bothered to breadboard some buttons? These are easy to plug in and go, although not many newer printers have one. :

Some other gems can also occasionally be found, but these are the most likely things. Amazingly, all you need to disassemble most printers is a big phillips screwdriver and perhaps some wire cutters - everything snaps apart quite satisfyingly. Since so many get thrown away this is a nice low cost way to get parts :)