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All Aboard!

I'd like to add my all-time favorite LEGO piece:

• x870cc02, THE BLACK WHISTLE!
  

The white brick is a microphone. Attach it to your train's motor and BLOW THAT WHISTLE! Depending on how short (or LONG!) your whistling was, the train would go forwards or backwards.

• Real-world use: Not only did it drive your train round the bend, but your parents too...!
  

One obvious answer is all components of the power functions family.

Lights can be used as lights, motors can be used as motors.

The holes in Lego Technik work very well as holes.

By which I mean, I have built very successful motor-driven cross slides for small machine tools using Technik pieces.

When paired with nominally 4.8mm metric ground rod (which tends to have a negative tolerance) the 4.8mm holes are a nice sliding fit. Imperial 3/16" rod at about 4.76mm also works, but is a bit sloppier.

Although the hole is a little undersized for tapping M6 (5.0 required), the soft plastic easily allows a taper tap to be run through. The 1mm pitch of the thread, together with the hard metric 8mm width of the brick means that the start and finish phase of the threads is identical. If an already tapped brick is pegged as a guide to a new brick, then (a) the guide will help ensure that thread goes in straight and (b) has the same phase in both bricks. I mark a guide brick, so there's no cummulative error in creating new tapped bricks.

A piece of 6mm studding can then be used as a lead screw. It can be fitted concentrically to Technik shafts by locknutting it to a tapped brick, then pegging the brick to a 40t gear.

I once published a paper in a peer reviewed scientific journal using data gathered using a remote translation stage constructed from Lego Technic.

I could only get ~ +-1mm accuracy out of it, where the proper stages were more like +- 0.1mm, but it was good enough for a proof of concept and the proper stages were on back order.

Of course many people would say that an X-Ray Physics lab is not the real world, but that's another matter!

Those shoehorn pieces worked. Technically, all of them serve their intended purpose as bricks. The ropes function. The ball shooting contraption present in the Lego Ninjago dragons (and other sets, I bet) functioned, as well as the spring-loaded cannon pieces. The axles technically worked (but not very well in actual cars).

I suppose several of the tools in Minifigure utensil wheel theoretically could be used in certain circumstances. I.e. you could hit things with the hammer, it would probably break before you got anything useful done with it, but it can still be said to have it's functionality.

Disclaimer: this is cheating a bit.

All of them!

I mean - even the most simple lego brick serves as a brick, so a creative mind can devise a real-world use for almost any brick if you have enough time, money and bricks at hand.

Gears function quite well for creating mechanical devices such as functioning clocks:

Design by KEvronista

Naturally you can use an electric motor for more accurate timekeeping.

The web pieces made of flexible string are perfectly serviceable real-life webs, even if they are too small for most applications.

Of course the LEGO brand magnets are working real-life magnets as well.

Some sets have included large air bellows that could be compressed manually (i.e. hit) to supply a burst of air to launch a projectile or a racecar.

The light-up bricks are naturally serviceable as tiny flashlights.

The rechargeable batteries are usable outside of the LEGO world as batteries.

The large sawblade used in some constructible action figure lines (for example Bionicle) and System lines (for example Power Miners) with the right setup could be used as a table saw to cut through paper or other materials of comparable strength.

The parachutes of the 2019 City Police sets are advertiesed as working parachutes, so I'd expect them to be ... working parachutes, at least for loads comparable to minifigures.

Almost all LEGO propeller pieces are functional propellers (having pitch and accepting an axle), with admittedly terrible performance compared to purpose-engineered examples.

The old-style 9V and newer PF electric extension wires could be used as real wires.

The electrical switches (or polarity changers) are working too.

In the same vein, the pneumatic components (pump, tube, valve, cylinder, tank, manometer) are working as well.

The Technic shock absorbers (springs) would apply too, some are even adjustable.

Naturally, all Technic gears are working gears, even if they have suboptimal shapes and materials compared to real-world examples.

The LEGO City mugs can actually hold tiny amounts of liquids.

The boat hull pieces that are advertised as being able to float, do of course float.

Possibly the most boring example of all: the weighted brick does indeed weight quite a lot due to a metal insert and is thus usable as counterweight.