The odor is ozone. It is created when an electrical arc splits oxygen atoms and they recombine into O3. You can smell it around an electric motor that is running or during a close electrical storm.

Oops, you can't split oxygen atoms without a particle accelerator or nuclear reaction. I meant you split oxygen molecules to get the ozone. I don't want the chemistry types jumping all over me for that minor technicality.

Ozone and the stench of burning insulation are the most common 'electrical' smells you will find.
Ozone has a characteristic sharp odor and is mildly irritating.
The O3 molecule quickly breaks down throwing off an oxygen atom that is VERY reactive.
Exposure to high enough levels of free oxygen can cause burns at room temperature.
It is generated every time an arc occurs in small quantities.
Universal motors, model trains, model race cars, etc. with the sparking from normal operation can make enough to become detectable.
Run an electric drill for a few seconds, then smell near the vents for the motor and you will get a whiff of ozone.

Arcing also causes a lot of heat (many thousands of degrees).
This can lead to burning of insulation.
Depending on what kind is burning (and how hot it actually is) the smell can be mild to pungent.

Either smell from permanent wiring indicates a problem.
Snap switches do not produce enough ozone to be noticed.
A slight smell might be present after a breaker opens from an overload (particularly a short circuit) in a closed panel.

I worked for a company that made ozone generators for water treatment. Actually, ozone is a fairly sweet smelling gas and not pungent at all. When lightning strikes during a storm you get that 'fresh', Spring-like smell. That is caused by the lightning's energy creating ozone. Ozone has a half life of around 20 minutes so it isn't around long, but it is VERY corrosive and you should not breathe it in although you want to because of its pleasant odor. Although many people swear by ozone for indoor air quality, I can tell you it is a very bad idea. It destroys carbon-based tissue. Can't see how that is a good idea to breathe in...

I would describe the 'bad' electrical smell as burnt metal with a hint of burnt plastic.

I'm a little surprised none of the professional sparkys mentioned the folly of letting the blue smoke out of an electrical device or component. ;)

Do you mean the magic smoke sometimes referred to in electronics?
"All electronics work on magic smoke, if you let this smoke out, the electroincs will cease to work"

Electrical smell, how do you describe it well.. I seem to think of those little slot cars i used to have as a kid, that smell you get running them 'round the track after the little motors heat up.. if you've never shared that experience, perhapse we do not have a frame of reference..

If ozone is unpleasent, why is it used as an air filtration method?

The smell of ozone is not unpleasant, but breathing it in is. It is used because it can work, but many do not consider it safe for human exposure even in modest concentrations. It is really tough on people with asthma. However, since it has a half life of ~20 minutes it doesnt' stick around long. e.g. a 20ppm concentration of ozone is reduced to 10 pmm in 20 minutes, 5 ppm after 40 minutes, etc.

Technically, ozone is not used as an 'air filter' per se. It does not work like a furnace air filter or coffee filter which forms a physical barrier. Instead, it reacts with different particles in the air and environment. Mold and mildew cells are 'destroyed' because of its corrosive behavior. Double bond carbon molecules (aromatics for instance) are broken up to eliminate their unpleasant odor. Dust particles react electrically to the highly charged ozone and 'fall out' of the air rather than float. Thus it is not really an air filter, but more of an air treatment method.

"The smell of ozone is not unpleasant, but breathing it in is."

Even the smell is irritating to many people.
Most of the electrostatic air filters come with a method to reduce the voltage to prevent excessive ozone production.

At VERY low concentrations it might be "a fairly sweet smelling gas and not pungent at all".
At higher levels it causes burning of the eyes and mucous membranes and is very irritating.
Free oxygen produced by the decay of ozone s VERY irritating.
It will oxidize just about anything it comes in contact with, and is used to sterilize things.

Hey Brick, the smell has nothing to do with what it does. That's a separate property. Whether its smell is sweet or if it was pungent, yes it is still very irritating and can burn eyes and nose. Just because arsenic poisons smell like almonds doesn't mean people are going to like it... I like my sister, but it doesn't mean she isn't irritating (sometimes). :-p

"Hey Brick, the smell has nothing to do with what it does."

Never said it did.
It is an irritant.
Different people tolerate different levels, but everyone finds very high levels to be a problem.

It is used to purify air (and even water) since it will oxidize and destroy bacteria, and even viruses, under the right conditions.
They are 'burned' at room temperature by the extreme reactivity of free oxygen.

Ozone generators for air purification are normally located far enough from outlets to allow the gas to react and dissipate before it is discharged in to the living space.

Ozone smells like a circus weasel or a swamp donkey depends if you are north or south of the equator the ozone spins the other way....stick you hand in front of the air discharge vents on a room Ionizer system...that smell that is on your skin....ozone....and because I am north of the equator I would say a circus weasel.

Smells like melting plastic and the smell burns your nose and eyes. Just went through it today.