Washing machine: probably not useful. The motor is under electronic control and has multiple windings, making it difficult to adapt to other jobs. Solenoid valves can be re-used elsewhere sometimes (but mains+water!) unsuitable for petrochemicals. Likewise the pump, but those in appliances are not self-priming - they almost always need to be below the liquid level or they airlock when you turn them on.
Tumble Dryer: has a small, highly geared motor (belt driven). Possibly useful if you can get suitable belts (the motor's "pulley" is usually squared-off grooves on its shaft, for cheapness, like my bandsaw and table saw belts).
Domestic dishwasher: just pump(s) and solenoid valves - no motor for moving things (that's done by water pressure). The commercial ones (Hobart, etc.) may have conveyor motors, etc., but I've no experience of them.
Hoover: modern ones have motors that aren't easily adapted, apart from the "Dalek" types (Nilfisk, Numatic, Earlex, etc.). These motors are powerful and self-contained, BUT they need forced air cooling and they're not very efficient. The same motors are fitted to many portable dust extractors. New spares are easily sourced but not cheap: £50-100 is typical, and often it's more than a new machine because of pipeline costs.
Vehicle starter motors: For years the staple diet of cavers wanting to build strong masonry drills for use underground, until modern battery drills arrived. You welded a masonry drill straight onto the shaft and ran it from a gel 12V battery. Hi torque (high current!), electrically safe (in a cave!), but won't run reliably for extended periods. They might also be adapted to serve as dynamos (DC generators), although probably not as efficient as...
... vehicle alternators: generate an AC wave that's rectified in a diode block to approx 13.5V for the vehicle (same for lorries but read about 27V (at a guess). No good as a motor!
Fridges: disposal of refrigerant is an issue, but fridge compressors are good for vacuum pumps for veneering.
Central heating pumps: The external ones (outside the boiler) are handy: they are designed to run hot, so if you find a working one it will probably stay working for ages (make friends with a plumber!). They often have a simple speed control built in. They are reasonably tolerant of some solids in the liquid (use a filter for large lumps, but sand-sized particles shouldn't cause immediate failure). They're designed to be quickly replaced too, so if you use one and it fails, you can easily drop in another. Downsides: They are pumps only: the rotor is immersed in the liquid being pumped (usually water), and relies on this fact both for lubrication and for cooling. The ones I've dismantled haven't had proper bearings - the rotor 'floats' in the middle of the field windings, surrounded by the liquid it's pumping. They're also an odd compromise between pressure and volume flow - works for central heating but can be a PITA for other applications. During the dry spells of the late 1980s I used one to empty the bath in our basement bathroom onto the garden. It would just about drive a sprinkler, but not well - it didn't have enough pressure. You can get 'negative head' pressure switches that will sense water demand and operate the pump, but when I last looked they weren't cheap, and CH pumps will allow flow backwards when they're off as the rotor can turn freely in either direction.
More on central heating: the motorised valves have small mains induction motors in them:
These can be handy. They're small, fairly quiet, and work in one direction only (not reversible) but they come with a high ratio gearbox fitted, and they're designed to be stalled for long periods while the valve is open (it closes with a spring, which the motor stretches to open the valve). They have plates to mount them (as you can see they're screwed or bolted onto the mounting), and spares are commonplace. I note in passing they've shot up in price recently - were around £6 three or four years ago!
Other stuff: Kenwood "Chef" food mixers have mains motor systems that are variable speed with reasonable torque (but there's a lot of gearing-down in the machine). On the down side, it's the motor control that tends to wear out or break first.
Hand-held modern power drills have decent motors at varying low voltages. They're intended to be variable speed so might be adapted.
Older appliances and machines were designed with the motor as a separate component, often bought in from the likes of GEC, and fitted as one unit into a casing or onto the machine. Good examples are electric air fans, large air conditioners and garage air compressors. Those will mainly be single-speed motors and adaptable for other uses.
Modern designs save cost by incorporating the motor into the device. So a modern hoover doesn't have a separate motor inside: the "casing" for the motor is made from the outer shell of the machine, even down to bearing carriers being moulded into the plastic. These are almost impossible to 're-purpose'.
Note too that induction motors are designed to run at a single speed. anything that changes the speed the things run at is usually external to the motor, for example a set of pulleys or a gearbox on a lathe. They're quieter than brushed motors, but the latter are what you need if you want to control the speed.
If you recycle motors from old mains appliances, watch for overheated and thus dangerous wiring (goes brittle and the insulation drops off), worn out bearings, etc. Unless it's just for fun, and you know you can get a ready supply of identical motors as drop-in replacements, you may be better off using something new.
Remember that generally, like cars, each design of appliance has its own failure modes. "They all do that, sir." And they will. If one hoover died when the top motor bearing failed, every motor you blag from that design is likely to have the same weakness.
It's one reason why you might have more success with old cordless power tools - they are usually varispeed, and it's the battery that dies, not the motor.
Well, that lot should get you thinking!
E.
PS: Small fans are almost always "squirrel cage" induction motors (Google it). Even those for 12V supply are like that, and they need 50Hz AC to operate. These are inefficient, low power motors (generally speaking), used mainly because they're quiet and cheap to make. You're highly unlikely to be able to adapt them to anything, especially as the failure mode is almost always the cheap bearings!
PPS: The mechanical systems from computer printers can be a very good source of bits, also big flatbed scanners and plotters (if you can still find any of the latter). Usually 12V DC motors, they have shaft encoders, and can be moved very precisely, often bi-directionally. The ones from bigger machines, e.g. departmental-sized copiers and printers can be quite chunky. Driving them, however, is complex.
