I just had some fun with a couple of tiny motors from 'disposable' toothbrushes. I made them into a little light following 'Snailbot' no Arduino, just two transistors and two phototransistors and a rechargeable button cell.
But it got me thinking about what else is reusable from dirt cheap consumer electronics and electricals?
Plenty, if you have more time than money.
The biggest issue with most consumer electronics (which makes them so cheap) is the switch to mostly SMT processes. If you can set yourself up an SMT disassembly process and have the time, you can gather up a good collection of components - the only issue being that most of the time, some of the components may be difficult to impossible to figure out what there specs are (if they are unmarked, mainly). You might also have to invest (or build) some special test equipment to measure certain components (ie, tweezer probes for caps/resistors).
Great sources of components for robotics can be had from old toys, especially remote controlled cars and such. Old VCRs, TVs and DVD players, besides the "standard components" that you expect in them, also are nice sources for the IR receivers (and get the IRLED from the remote, of course).
PC printers and scanners have fun parts in them that could be useful for robotic arms and lightweight CNC setups. Plus, in a scanner, theres always the linear CCD (if you can figure out how to hook it up) - such a CCD could be turned into a laser rangefinder using a laser (perhaps from a CD/DVD player?). There's also opto-interruptors to be found in both...
If you need solar cells or LDRs - you might look into old or cheapo garden lights; people sometimes throw them away when the rechargeable batteries in them die, but the solar cells and LDRs can typically be salvaged (they won't be in great condition; you might have to do some or a lot of cleanup work!).
Other items I have seen that people get rid of cheap (or even possibly throw away) include old Roombas and Robosapiens; I've seen both sold used for under $20.00 (I once picked up a Roomba from Goodwill for $5.00). Talk about a wealth of parts for robotics! Plus the Roomba has a few Sharp IR like sensors...
Old computer mice can be interesting - old ball mice have the encoders in them, of course, which are easily interfaced with; scroll wheels are also a form of encoder. Old optical mice can sometimes be used as small low-res cameras (64x64 pixel) - maybe a small robotic vision system? Then there are old/broken Playstation and similar controllers - great source of analog joysticks...
Plenty of stuff...
Plenty, if you have more time than money.
No kidding! I have collected a lot of parts from discarded electronics. My favorites are power transistors/MOSFETs, transformers, through-hole capacitors and some low-pin count ICs.
And if it is a SMD part I really want, I'll spend the time to carefully remove it.
PC printers and scanners have fun parts in them that could be useful for robotic arms and lightweight CNC setups. Plus, in a scanner, theres always the linear CCD (if you can figure out how to hook it up) - such a CCD could be turned into a laser rangefinder using a laser (perhaps from a CD/DVD player?). There's also opto-interruptors to be found in both...
It is pretty cool. We threw away so much stuff prior to getting back into electronics and programming. We still throw a bunch away but at least I can pick what I want out of the pile for free 
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Old toys can often be salvaged outright for bases for robotics projects. I am building a autonomous robot currently that is based on an old "RAD robot", as a bonus I could salvage the original H bridges by cutting the original PCB in half ;D .
as for other things, lets see:
Old LAN cards and gameboy cartridges will often have SRAM chips in them.
VCRs often have nice motors.
Polaroid cameras have easy to interface ultrasonic rangefinder modules that have a longer range the most commonly available ones.
Old camera flashes can be a useful source of pulsed high voltage.
old pagers and mobile phones have nice small motors.
that is all I can think of right now.
A friend and i had a lot of fun with an old photocopier. Dismantling that is.
Result was various sizes stepper motors with (throughole) driver, multiple brush-less motors with driver/controller boards attached, a solid-state relay 220VAC 16A, a lot of photo interrupters, light to frequency converter (on a separate board with opamp thingy and all) and the list goes on.
Trick was to measure supply/signal voltages and trace wires prior to dismantling. After that the datasheets were just confirming what we traced/found out.
Jeroen
Plus, in a scanner, theres always the
CCFL... Nice bright white light for soldering etc. Make sure you don't zap yourself with the high voltage though, it husts a bit. ;D
Mowcius
I like going to thrift stores, you can usually find older stuff there, alarm clocks are good for their displays, phone machines are good for their amp chips and other tidbits etc
Dvd burners= powerfull red laser diodes can cut plastic and burn wood (depends on optics) BE CAREFULL YOU CAN LOST YOUR EYES!
Dvd burners= powerfull red laser diodes can cut plastic and burn wood
Is that indeed a fact? My BS alarm just went off. 
However I do indeed go with the protect the eyes thing for all lasers no matter what their power rating is.
Lefty
oh if your very patient you can burn a hole though some thin black plastic, maybe scorch some wood
Dvd burners= powerfull red laser diodes can cut plastic and burn wood (depends on optics)
true...
we have a huge junk box lying here in the office... with most of the above in some state of dismantle... all off a sudden, we need something, we delve into the box and try and get something that we can work with...
I've been personally blessed with the mother lode of all mother lodes . . . my employer has been physically downsizing.
I work in automation and a year ago, we emptied out a small facility a block away and put a lot of their spare parts in storage at my facility. Now we are in the process of moving from the two remaining facilities to one facility that is big enough to house both divisions.
The result as we clean out the storage shelves has been an incredible blessing of sensors, shielded sensor wire, pneumatic cylinders & valves, Lexan polycarbonate sheet, Bosch aluminum strut and hardware.
Perhaps the best pick has been a working vacuum pump.
The hardest part has been sneaking all this stuff into the house and down into the workshop without my wife seeing more junk coming in.
I have also done well doing a little scavenging for old VCR's, printers, etc on “garbage night” in years past.
Unfortunate for me . . . but better for the environment, in my area all E-waste must go to a special recycling centre . . . however since all incoming E-waste is broken down and ground up for automated material separation, they will not let people like me come in and pick through it; claiming liability issues and such. Damm lawyers!
The hardest part has been sneaking all this stuff into the house and down into the workshop without my wife seeing more junk coming in.
Yeah sounds about right for most people. 
they will not let people like me come in and pick through it; claiming liability issues and such. Damm lawyers!
Yes it's a right pain. Same with the local tip. Freecycle has proved to be ok for it though but most people don't post up broken stuff, you have to specifically ask for it.
Mowcius
I made a little project a while back (not Arduino) that had a 'fake balloon popper. Regular laser pointer hits the balloon acting as a diffuser for a phototransistor behind it in a little box that uses a mosfet to drop a few amps through a small resistor taped to the back of the balloon. After a few seconds of lasering from the other side of the room the balloon pops!
Much the best way of balloon popping!
Another discovery.
Ribbon cable is expensive.
reclaimed IDE hard disk leads are a great source of cheap ribbon cable.
I took the motor and fan from my wife's old hair dryer to make a vent fan for solder. I still get light headed though [smiley=grin.gif]
Old phones have a lot of good caps, transistors, resists, etc
Got an i2c EEPROM chip I need to take off a broken TV tuner card.
Also got a load of other circuit boards in my 'to be dismantled' pile.