Yes! there is such a thing as a variable capacitor, although as Palindr=E2=98=BBme said i've never seen a value above the pico farad range. Variable capacitors are much more common than you might think. They are the most common way to tune small radios. The next time you get your hands on a $5 radio or some price in that area open it up, i can almost guarentee there will be a small translucent box with a rod sticking out of it connected the tuneing knob. Turn the knob and look through the plastic box, you should see metal plates moving inside.
this url will take you to a site that tells you how to build your own (notice it's a radio site)
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This one will take you to site that briefly describes variable capacitors, you'll have to scroll down almost to the bottom
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This web address will take you to Froogle, google's selling site
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Enjoy!