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.
The treasure chest, large barrel and letterbox pieces are tiny, but functional storage items.
Some City sets include working car jacks.
LEGO has produced mirror pieces with stickers that really reflect light and of course all transparent window panels let light through. On the same topic, many door elements have working hinges and the tiny fences could be used to separate tiny gardens and to keep tiny animals from crossing over. (Caveat: almost all animals that are small enough that can't just step over it can probably climb over or fit through the holes)
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.