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Well you can get a cup size immersion heater:

http://www.amazon.com/Instant-Immersion-Heater-Portable-Beve...

For about $9. We used to have those growing up.

But those can be very unsafe. (Or at least those designed and built by the Soviet industry were...).

I used to use these. Convenient on the go, always amazed my fellow travelers. But then I read an Amazon review where the guy used a multimeter to demonstrate that his model conducted electricity into his cup... and I decided to stick to normal kettles from then on!

If you pass an electric current through a metal coil you will create a magnetic field. A magnetic field will create an electric field. So it isn't surprising a heating coil produces an electricity into the cup. What is important is how much and where.

Most people, I assume, are smart enough not to put there finger into a cup of hotwater to find where the electric current runs through.

I had the additional problem of using a metal Klean Kantern to boil my water, so it was easy to forget to not touch the outside. (Mine never shocked me, but I get the feeling that these things aren't exactly made with care...)

Or (don't)build it youself! https://www.youtube.com/watch?v=aTejx32xVgc Could be the most UNsafe use of 'safe' razor blades.

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