I noticed when walking home the other day that the high voltage (200kv I believe) lines running through here were hissing in the rain. What is causing them to hiss?
High-voltage partial discharges across the insulators.
I was taught that this is due to the Corona effect. Basically, the power lines ionize the air around them, causing audible hum, along with havoc in the EM spectrum. This is why really high voltage lines and transformers will sometimes have a slight aura around them.
Generally, the effect is undesired, because it robs the transmission lines of energy (the hum/light/heat dissipates energy), so a lot of equipment is manufactured to try and stop this effect.
The Wikipedia article will do this subject much more justice than I can.
Good stuff in the answers so far, but I work with power lines and want to put in my two cents.
This isn't technically a partial discharge; occasionally you may be hearing the crackle of what is usually called a corona discharge. I admit the phenomenon is related, but it is not the same.
See, all uninsulated lines show corona. Its just not a big deal until you're dealing with a pretty high voltage. As the voltage goes from a very big positive to a very big negative, the air around it gets ionized, so about 50 or 60 times a second, it switches direction. This is the normal mains hum discussed in another answer.
Water is much, much heavier than air, and it ionizes just as easily. So on a rainy or humid day, the corona is pulsing with water in it. This gives it momentum, so the heavier water particles travel out farther. But they themselves are ionized, which means they can ionize more air than the line could normally reach on its own, and ionized air is conductive.
And there's almost always 3 of these lines pretty close together. The sound you're hearing is a million teeny tiny electrostatic discharges from all the charged up water particles interacting with each other with nearby lines or grounded objects. This is actually the worst time to be anywhere near them; the air is supposed to be their insulator, and at that moment it isn't working as well.
Occasionally you might see a full corona discharging with the naked eye; it looks like a tiny bit of lighting crawling up the line. If it gets really bad, you'll see a momentary line to line or line to ground short, which looks exactly like a real lighting bolt, just not from the sky.
They'll actually do it when it's not raining too. It's called mains hum. Power lines carry AC voltage at either 50 or 60 Hz which is at the low end of the audible range of most humans. In the presence of an electromagnetic field (like the one generated by AC power), the molecules of ferromagnetic materials (the metallic conductors inside of power lines) will not only try and align themselves with the field but sometimes change or distort their shape if the applied potential is strong enough. This alignment/change in shape can cause collisions between the molecules comprising the power-lines which, given enough of them, can be heard by an observer.
My wild guess would be that the hanging water droplets might be causing corona discharges. Corona is usually worst around points of sharper curvature, where the electric field gradient is most intense. Higher voltages, like the 200kV you mention, would make this more pronounced.
There are several causes but the most common is high voltage discharge along surface contamination on the insulators. This has a characteristic "hissing" noise, whereas most other phenonomon generate noise primarily at the line frequency or 2nd harmonic.
During dry weather dust builds up on insulators and it always contains some quantity of salt. When weather gets damp, enough water gets into the mix to allow it to become conductive, so small quantities of current start flowing. Once that happens any organics in the current path quickly become carbonised and the arc becomes more-or-less permanent (heavy rain will wash the insulators clean and the cycle then starts again)
The noise peak for hissing discharges is ultrasonic, so detecting where the actual arc is located is done using a small parabolic microphone with a viewfinder. (see http://www.arrl.org/files/file/Technology/PLN/Ultrasonic_Pinpointer.pdf for an example)
The hissing you hear is the least of it. Radio hams HATE arcs because they generate a lot of radio interference - so they tend to be the most enthusiastic purchasers/builders of ultrasonic arc locators.
Dust buildup is the primary reason why high voltage insulators aren't smooth - the typical "stack of discs" design makes it very hard for enough dust to build up to let current flow under damp conditions, so dust-based arcing is usually found only on 11kV or lower tension poles, where the insulators are a much simpler design.
Corona effects are usually only found on very high voltage lines (over 220kV)