At one point in time, Tesla coils were actually used for things like wireless telegraphy and electrotherapy, but as technology advanced, they shifted to a slightly more enjoyable purpose—entertainment. What's even more entertaining than using a Tesla coil? Building your own.
One of the best portable Tesla coils out there is this mini acrylic version by Daniel Eindhoven, aka TeslaCommander. It's made almost entirely of acrylic plastic, minus the steel sphere, and copper wire and tubing. When the lights are out, the whole unit lights up, not just the high-voltage lightning-like plasma flying from the top load sphere.
If you're familiar with electronics and Tesla coils already, this shouldn't be too big of project. The hardest part is probably shaping all of the acrylic sections to the right dimensions.
If you want to give it a try, Eindhoven has detailed blueprints, parts list, instructions, and more photos over on his site.